Learn Finnish: Onko tämä yksilö luonteeltaan vaarallinen? - Is this individual dangerous by its nature?

This uses the ablative of respect. It's built from the noun luonne (character/nature). To specify what exact aspect makes the individual dangerous, Finnish uses the ablative case -lta (from) plus a possessive suffix, which is the 3rd person -an (its) here. Literally, it means "from its nature", but it functions like the English "by nature" or "in character".

Is it possible to drop the possessive suffix and just say luonteelta vaarallinen?

No, the possessive suffix is mandatory for this grammatical structure. The ablative of respect always needs the possessive suffix to link the quality back to the subject. Without the -an, luonteelta would just mean "from the nature/character" in a directional or source sense, which doesn't work here.

Why does the word luonne change to luontee- before the endings?

luonne belongs to the group of nouns ending in -nne (like tilanne or huone). To form the stem for case endings, the -nne changes to a strong grade -ntee-. So, luonne becomes the stem luontee-, to which we then add -lta and -an.

Why use yksilö instead of henkilö here?