gratus

Gratus, grata, gratum is first and second declension and takes a dative for the person pleased: gratum mihi, welcome to me. Comparative gratior, superlative gratissimus.

It points both ways at once, which is what catches people out. Of a thing it means pleasing, welcome, acceptable; of a person it means thankful. So "res grata" is a welcome thing and "animus gratus" a grateful mind, and ingratus reverses both halves, unwelcome and ungrateful. The noun on the same root is gratia, "favour, thanks, grace", giving "gratias agere", to give thanks, and "gratis", free of charge, which is an ablative plural meaning "for thanks alone". Spanish gracias and Italian grazie descend directly from it.