It’s a common misconception to call a single member of this race a jinni. These psychically-linked beings have no concept of singularity. They are always connected to one another and never alone.
The jinn is a very, very old and very, very powerful race of beings from a hellish, fiery dimension. In their natural state, they are said to be comprised entirely of smoke and fire. This is likely true, considering that almost any time a door opens to their realm, the area and any unfortunate bystanders surrounding said door are instantly incinerated.
The jinn, as a race, are neither inherently benevolent or evil. Like us, they are free to choose their own path. One is just as likely to grant you mercy as to set you ablaze. It really just depends on how grumpy he is. Of course, even the gentlest of jinn won’t lose sleep if you’re caught in the hellfire as they step over from their universe. We are, after all, only an inferior race.
(Note: I’ve found no evidence that the jinn have ever been associated with wishes.)