Regardless of whether it is a No Mercy, True Pacifist, or Neutral run, some fans prefer to have Chara be more involved in the story of Underfell by appearing alongside Frisk in their journey, either with or without their knowledge.
Chara's death and what happened with Asriel and the humans, often playing out as its shown in the canon Undertale, is often pegged as being the catalyst for the Kill or Be Killed motto of Underfell, their death and the subsequent grief it caused Asgore fueling his edgy behavior. Some fan depictions have the monsters already acting cruel or edgy before Chara falls, however.
Chara is a human child of indeterminate age (though some see them as a child around the age of ten) and gender, with a skin color typically shown as being lighter than Frisk's. They are illustrated as having pink cheeks and red or brown eyes, as well as light brown hair often drawn in a bob or bowl cut.
Chara's clothing is usually drawn as a red sweater with a black stripe, black pants, and brown boots, however, popular interpretations of their sweater colors vary more widely than in Frisk's case.
Chara's personality is up to interpretation, as is the case for their Undertale counterpart. However, the most common depiction is for them to be a "good" person, trying to save monsters and help Frisk through the Underground. With these depictions, some fans write Chara as being bullied by Asriel before their untimely death. At the end of a True Pacifist run, Chara is sometimes shown as appearing to the player and congratulating them on exercising mercy.
Other depictions have Chara act ambiguously sadistic and cruel when they appear, and making them a clear-cut bully to Asriel during their life.
Chara's personality as a child is often unclear. Their behavior is sometimes forthright, immediately lending Frisk aid, or obscure, appearing as a foreboding figure through their journey. There is not any personality, story, or role that Chara is given in different interpretations that everyone agrees on.
In general, Chara is a very controversial "chara-cter" that has many different personalities and interpretations in Undertale and its fandom and AUs.