Learn Finnish: Nämä pojat ovat kaksoset, mutta heillä on täysin erilainen luonne. - These boys are twins, but they have a completely different character.

To contrast the twins directly using the "to have" structure, you must put both pronouns in the adessive case. You could say: ...mutta toisella on tällainen luonne ja toisella täysin erilainen (but one has this kind of character, and the other has a completely different one).

Why is erilainen luonne (different character) in the singular when the sentence is about two boys?

In Finnish, it is very common to use the singular when multiple people each possess one of the same thing (such as a personality, a face, or a life). While you could technically use the plural erilaiset luonteet, the singular sounds completely natural here to express that each boy has his own character.

Why does the word for "completely" end in -in (täysin)?

The -in suffix is the standard way to turn an adjective into an adverb in Finnish, much like "-ly" in English. Here, it changes the adjective täysi (full) into the adverb täysin (fully or completely) so it can modify the adjective erilainen.

Why is kaksoset in the nominative plural instead of the partitive plural?