At the end of "Spirited Away", we learn that Haku's full name is Nigihayami Kohakunushi.
I came across a discussion about the possible kanji writing for this name.
Because the "kohaku" comes from the Kohaku River, of which he is the river god, the consensus seems to be that this is the Amber River, and so kohaku can be written as
琥珀. (It could also be 古白 "old white" or 小白 (small white") but I like amber better.)
And "nushi" is 主, "lord".
So "kohakunushi" is in kanji 琥珀主.
The meaning of 邇芸 is apparently "gentle", so邇芸速 is gentle-swift
The "mi" part of Haku's full name is just guesswork. I didn't like the proposed ones in that particular discussion: 美 or 巳; but in another discussion, the suggestion was 水 (water) and that makes more sense to me, we get 邇芸速水 "gentle, swift water". With suggestions for kohaku as above, Haku's full name in kanji would be
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!