Chara was originally the Acolyte of faith. However, after Sans failed to reap Chara, darkness (powers of death) had passed to Chara. Since that day Chara walks among the realms (both mortal and godly) as a corrupted vessel of darkness. When Asriel died he returned as Flowey (the same satanic flower). Chara doesn't know that Flowey is Asriel and neither does he.
You need to SAVE Asriel/Flowey to be able to SAVE Chara.