Alphys

Dr. Alphys is a minor protagonist in Avengers Endgame. She only appears in Acts II and III. She also created the Amalgamates by accident in an experiment to make monsters' souls be able to persist after death.

Appearance
Dr. Alphys is a yellow, reptilian monster with protruding front teeth. She has triangular spines on her head and wears a lab coat.

Story
Dr. Alphys is first seen on Act II, where she is held captive with several of the Avengers, Mettaton, and The Amalgamates on Thanos's Sanctuary Ship. She helps the Avengers escape the ship and rescue "Aqua" (Himiko Toga in disguise)*. Past-Alphys is killed by Evil Bowser. However, Alphys is brought back in Act III during the Final Battle with Undyne and Mettaton. Ironically, Past-Bowser killed her in the 2012 timeline, but Present-Bowser brought her back when he snapped. She attends Deku's funeral.

* Alphys was unaware that Aqua is actually Toga, and she wasn't attempting to sabotage the Avengers.