In any case I just want to make clear that the fact that these are not normal people.
If I’m not mistaken, まとも 's kanji is 真面. however, when typing まとも on a Japanese keyboard I don’t see this pop up anywhere near the top of the search results. Is it usually spelled with hiragana? How did 面 get the pronunciationとも? Is it actually its 訓読み pronunciation?
Type in まともな and press the enter key, so you’ll see 真面な. まともな in hiragana is probably more frequently used than 真面な in Kanji.
There are two possible etymological theories.
真つ面（まつも）was the original form, and つ in the old Japanese expression is equal to の（of）in the modern Japanese. We still pronounce 面 as も such as 水面（みなも）= the surface of water/lake/pond. 真つ面 used to mean “the front face”.
真艫（まとも）was the original Kanji. 艫 means “the tail of a ship”. A strong wind blows against the ship. The condition was described as 真艫に風を受ける → 真面に風を受ける.