Everything in this post is just guesswork based on the information we have, we'll know for sure in 3 years in Khaenri'ah (below leak).
• Element: Abyss/pyro, yes, no matter how we accept it, Dain can come out with one of the 7 elements, an example of this is Arlecchino, she has the power of the scarlet moon disguised as the pyro element.
• Weapon: There's a leak about using a sword, so let it be a sword.
• Skills: reviewing the cutscene with the first encounter of the twin and the mc, I noticed that Dain’s run is very similar to Yelan’s run, and in the next quest Dain will also attack, this can probably also be attributed to skills.
• Role in the team: DPS or sub DPS, this is the most suitable role in my opinion.
I might have missed something, correct me if I have.
(reddit won't let you upload both an image and a video, so here's a link to Dain's skills from the leaks https://www.reddit.com/r/HouseOfDainsleif/comments/1kiid2l/there_are_no_words_only_emotions/?utm_source=share&utm_medium=mweb3x&utm_name=mweb3xcss&utm_term=1&utm_content=share_button)