He's literally one of the elvhen gods, but not inhabiting a body like Mythal is. There's a codex entry suggesting that he's really a god of "rebellion" rather than the negative connotations that the Dalish typically associate with him; myths claim he killed Mythal, but he apparently wasn't involved in her murder, nor did he seal her away, according to Abelas. The two also seem to have been good friends and possibly the foci might be symbolic of that if it in fact has any tangible relation to her.
If we take any accounts of Elvhen history by Solas - who likely is being selective in what he reveals- it seems the elvhen gods weren't as noble as one would imagine them in real life.
Uh...I didn't know that was even in question?
When I said "I'm not sure what Fen'Harel is in relation to the rest of the Pantheon/Forgotten Ones" I'm wondering where his "divinity" comes from.
Mythal and Elgar'nan seem to be the first of the Pantheon, with Falon'din referred to as "Eldest of Elgar'nan and Mythal". Dirthamen is known as Falon'din's twin-soul, but is not mentioned as a child of Mythal and Elgar'nan.
Andruil and Sylaise are mentioned as sisters, but not children of the Pantheon. June is known as their brother, but is also mentioned to be "self-made".
Ghilan'nain was raised to godhood by Andruil, which is interesting since it implies that their divinity is not inherent.
Then we have Fen'Harel, who may or may not have been raised to godhood by Mythal (he has her orb, after all) and has some connection to the Forgotten Ones, who we have almost no info about.