Placing an article before or after an adjective [duplicate]
"Such" is an adjective. Adjectives modify nouns or noun phrases. In the example "such" is modifying the entire noun phrase "a good person."
"So" on the other hand is an adverb. Adverbs modify adjectives (as well as verbs and other adverbs, although those two aren't relative here). "So" is modifying just the adjective "good"
For reference, see the Merriam Webster definitions for such and so.