Note: This profile contains spoilers for the game Undertale. Please do not continue if you have yet to play the game and do not want to be spoiled.
Alphys is a reptillian monster who acts as the Underground's Royal Scientist, a position previously occupied by the mysterious W. D. Gaster before his untimely demise, being appointed as such after creating a robot with a soul. In reality, Alphys and Mettaton's ghost were good friends (and both avid human admirers) before Alphys built Mettaton's robot body, allowing him to have the body he always wanted and her to get Asgore's attention. Alphys is shy and rather introverted, often stuttering and lacking self-confidence. She is also ridden with guilt due to past events and is often hinted to be suicidal despite her attempts to remain calm. Despite all this, Alphys thoroughly enjoys (most) human anime and manga, having amassed quite the collection from scavenging ones which have fallen into the Underground through Waterfall. She is close friends (and eventually something more) with Undyne despite their vastly differing personalities.
Despite being mentioned a few times by characters such as Papyrus and Undyne, Alphys does not actually appear until Frisk reaches Hotland's laboratory. Alphys will reveal that she's been watching the human on hidden cameras throughout their adventure, and has become fascinated with them. While stating she would love to help out, she mentions the "small" problem of accidentally having built a human-killing robot moments before Mettaton bursts through the wall and forces Frisk into a quiz show. Despite many of the questions being nearly impossible to solve in the brief period of time given to answer, Alphys will help out by discreetly revealing the correct answer with her hands. This goes smoothly until Mettaton asks a question about Mew Mew: Kissy Cutie (Alphys' absolute favorite anime), which causes her to loudly begin to blurt out the answers, alerting Mettaton to the situation. Simply to embarrass the scientist, Mettaton will ask the player who Alphys' has a crush on, despite her obviously being flustered by this. Depending on the player's answer, they can learn things such as Alphys' attraction to Asgore and her massive crush on Undyne. After this, Mettaton takes his leave.
Before leaving the lab to head deeper into Hotland, Alphys upgrades Frisk's phone, allowing it to utilize texting, the internet, and even store items in interdimensional storage boxes. After this, she quickly rushes off to the "bathroom". Throughout Frisk's trek through Hotland, Alphys will post random status updates on Undernet (something she completely forgot about signing Frisk up for, as well), allowing the child to better understand her as an individual. Along the way, she will also help guide them through puzzles and traps. Mettaton will show up several times in order to impede Frisk's progress, but each time, Alphys will reveal a new invention she installed in the phone (a jetpack, a bomb defusing device, and the ability to make Frisk's soul yellow and able to fire projectiles) which allows the child to escape safely. After the encounter with Mettaton just before the MTT resort, Alphys will confide in Frisk that she's felt like a screw-up for such a long time, and how being able to help made her feel important and confident.
While attempting to guide the human through the CORE, Alphys finds herself unable to help due to the massive device's changing layout, which deals a significant blow to her recently acquired confidence. Once Frisk manages to reach the end of the core, they once again encounter Mettaton, who reveals to them that he was only acting and that Alphys had essentially inserted herself into Frisk's story in order to make herself feel helpful. It was actually she who had been reactivating traps, disabling elevators, and having Mettaton "attack" them. However, Mettaton now has his own agenda, as he believes that Frisk is not powerful enough to defeat Asgore, and will simply give the king the final soul he needs in order to wipe out humanity. Because of this, Mettaton wishes to take Frisk's soul and leave the Underground, becoming a star on the surface along with preventing genocide. The human's attacks prove to be useless against Mettaton's tough exterior, but Alphys calls tells them that flipping the switch on his back may make him vulnerable. Doing so causes Mettaton to transform into Mettaton EX and the real fight to begin.
After defeating Mettaton EX, a concerned Alphys will rush in, either relieved that only his batteries died (if he was spared) or attempting to hide how distraught she is at his destruction (if he was killed). Alphys then lets Frisk know that a human cannot cross back through the barrier on their own, and that in order to leave, they must kill Asgore. She apologizes for one final time before fleeing. If Mettaton EX is killed, Sans' post-game phone call reveals that Alphys disappeared, likely having ended her own life due to feeling at fault for the loss of her friend.
After completing the game's neutral ending and defeating God Flowey, Frisk can now pay Undyne a visit if they had become friends with her prior to the last boss battle. Undyne, who is now living with Sans and Papyrus due to her current "living situation", will ask Frisk to do her a favor and deliver a letter to Alphys. While she says this is simply due to her hatred for Hotland, it becomes incredibly obvious that she is too shy to do so, herself. Upon delivering what is revealed to be a love letter, Alphys will mistakenly believe that it was Frisk who wrote it (due to Undyne not signing her name), which leads to the two going on the most awkward date imaginable...at the garbage dump. Undyne shows up, realizing it was a mistake to not deliver the letter herself, which prompts Alphys to confess to Frisk that Undyne is the one she really has a crush on. After some absolutely terrible roleplaying in an attempt to ease the tension, Alphys confesses to Undyne that she's lied about a great number of things, including her view of herself and the fact that anime isn't real. Undyne states that none of that matters to her, and it doesn't change how she really feels about her. Of course, Undyne still shoots her into a garbage can like a basketball and forces her to run one hundred laps with Papyrus, but it's the thought that counts.
After leaving the world's strangest date, Frisk receives a call from Papyrus, who tells them he sent Alphys home early and advises them to go check up on her. Upon returning to the lab, Frisk finds only a note from Alphys, telling them she's going to face her mistakes and to enter the door she previously referred to as the bathroom to find the truth. The door leads to an elevator which takes Frisk to a hidden area of the facilities known as the True Lab. By exploring the decrepit area, Frisk learns about Alphys' past experiments, her discovery of "Determination", the accidental creation of the Amalgamates, and the reason behind her guilt. After restoring power to the area, the Amalgamates begin to close in on the child, but Alphys comes in to calm them down, revealing they simply thought Frisk was carrying food for them. Alphys thanks Frisk for everything they've done, stating she's finally going to tell the truth and take the Amalgamates back to their old families, doing what's right as opposed to forever worrying about the consequences and living with her guilt. After she departs, Frisk learns one last shocking truth from a few previously inactive monitors; the result of Alphys' experiment to inject Determination into a non-living host with no soul happened to be Flowey. Almost immediately after, Alphys and the rest of the main cast play a pivotal role in the fight against Asriel Dreemurr.
Alphys is the only main character who is completely absent from the genocide route. According to Undyne, she told Alphys to evacuate everyone from Hotland in the scenario that she lost. After seeing the heroine slain at the hands of Chara, Alphys does as she was told and evacuates all civilians from Hotland. While she was able to get a great many monsters to safety, it's all for naught in the end when the demon who comes when people call its name ends the world.
Powers and Stats
Tier: Unknown. At least 9-B. At least 7-C with inventions, likely far higher
Name: Dr. Alphys
Classification: Monster, Royal Scientist, Otaku, Creator of Mettaton, Accidental creator of Flowey and the Amalgamates, Undyne's significant other, Mew Mew: Kissy Cutie superfan, Mew Mew: Kissy Cutie 2 hater
Powers and Abilities: Enhanced strength and durability, Likely Electricity Manipulation, Super genius capable of creating highly advanced robots, extracting and injecting Determination into subjects, and upgrading Frisk's phone into a bullet-shooting jetpack with interdimensional storage functions in a matter of seconds, Can ignore Durability to an extent via SOUL Magic, Danmaku, Technological Manipulation and Hacking, Resistance to Soul Manipulation (Can take hits at they soul like a normal attack)
Attack Potency: At least Wall level (Summoned bolts of electricity capable of blocking incredibly casual attacks from Flowey. Said attacks were capable of bringing Frisk to near death, at the beginning of the game). At least Town level with inventions (Created Mettaton EX, who is nearly as powerful as Undyne), likely far higher (Presumably also created Mettaton NEO, who is vastly superior to even Asgore and Toriel by an untold amount)
Speed: Unknown (While Alphys is far from a fighter, she was able to block casual shots from Flowey)
Lifting Strength: Unknown
Striking Strength: Unknown
Durability: At least Wall level, likely higher (Lives in Hotland, where even the heat of the outer area is capable of instantly turning a cup of water, including the styrofoam cup it's in, into vapor)
Standard Equipment: Numerous devices, her phone, soda, lots and lots of instant noodles
Intelligence: Super genius level intellect, Capable of building highly advanced machines, raising the dead via Determination, and turning an old phone into a highly advanced multi-purpose tool
Weaknesses: Can often lack self-confidence. She isn't much of a fighter either. Weak against murder intentions.