Pre-Death Chara is a normal human child by Undertale standards.
Post-Death Chara is present only as an energy (not really a spirit because they don't have a soul) during our journey in the Underground and, during a Genocide Route, they can occasionally and briefly possess Frisk which is their only means of interacting with the world. They were "revived" by the Determination of the player's soul interacting with their corpse.
Absolute Chara, their peak, is just Chara existing as an abstract force of energy lacking a body or a soul, who doesn't require either of those things to access any and all power they possess and use it to interact with the world, they can tap into a giant pit of accumulated Determination to perform feats on par with other high-DT users, such as Frisk. Both Frisk's and Chara's Determination comes from the Player's soul though I don't think the wiki currently acknowledges that detail. Absolute Chara is NOT bound by a physical form, Frisk isn't even there anymore actually, but they have access to generally all of the hax that Frisk would have due to Determination, because of basic Determination scaling.
Fyi, in Undertale, Determination is a power source that can physically be manifested and used. Generally speaking, only human souls have it, and there is so far only one Monster we know of who has developed her Determination naturally, that being Undyne. While it does correlate with the emotion called "determination", Toby Fox used some clever word play here. To be specific, Determination in Undertale is mostly linked to the will to live, however, when the character who officially named it that put the name in place, she wasn't thinking of the word determination which can be used interchangeably with words such as perseverance (which is a type of soul trait in the game), she named it that because of the main use, the ability to defy/cheat death, which can reshape someone's body when they are at the brink of death, cause regeneration, or even allow the user to reverse time if they die, among a PLETHORA of other uses.
Termination-"The end of something in time; the conclusion."
"DE-TERMINATION".
God I love Undertale.
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