More plausibly it's the Finnish form (also
Mika) of
Michael, or
Mike, the explanation given by Wiktionary. It's also a Japanese female name, most probably a use of the word for a Japanese shrine maiden, which literally means "god's child" (i.e. a child dedicated to or serving a god), in kanji 巫女 (miko woman),
https://en.wiktionary.org/wiki/%E5%B7%AB%E5%A5%B3#Japanese"From Old Japanese. Originally a compound of 神 (mi, “spirit, god”) + 子 (ko, “child”), as suggested by the alternative spelling 神子.
Pronunciation
(Irregular reading)
(Tokyo) みこ [míꜜkò] (Atamadaka - [1])[2][1]
(Tokyo) みこ [mìkó] (Heiban - [0])[2][1]
IPA(key): [miko̞]
Alternative forms
神子, 神巫 (rare), 巫 (obsolete)
Noun
巫女 (hiragana みこ, romaji miko)
An unmarried [girl] who serves at a Shintō shrine, helping to receive visitors and performing 神楽 (kagura) dances at ceremonies, among other responsibilities.
An unmarried [girl] who serves a deity as a medium to convey oracles."
The Kanji is possibly from the Old Persian symbol for a magus (one of the magi), through further development in Han Chinese ideographs.