I think a person who says, "you're the only one for me" is the kind of person who would cheat on you.
Whoever says stuff like, "Oh you're no.1" is the kind of person who would be unfaithful.
1) I think a person who says, "you're the only one for me" is the kind of person who would cheat on you.
「『私にはあなただけ』とか言う人に限って浮気しそうだと思う。」
cheat on で「浮気をする」
2) Whoever says stuff like, "oh you're no.1" is the kind of person who would be unfaithful.
「『あ〜君が一番だよ』みたいなこと言う人に限って不誠実なんだよね。」
unfaithful で「不貞な・不誠実な」
ご参考になれば幸いです！
A person who always tells you "you're the best," seems to be the kind of person who would cheat on you.
A person who always tells you "you're the best," seems like the kind of person who would cheat on you.
ご要望の形にするなら
A person who always tells you "you're the best," seems to be the kind of person who would cheat on you.
「あなたが一番だ」といつも言ってくる人は、浮気をしそうなタイプに思える。
という言い方ができます。
seem to be の代わりに seem like を使うこともできます。
「〜のように見える」「見た感じでは〜に思える」というような意味合いです。
日本 
