Funny that he got pulled out to work with Pantheon before he finished with Garen.
My timeline was Pantheon -> begin Sett -> begin Garen -> move to TFT (which is what I'm still doing), handing off Garen to GreaterBelugaWhale and Sett to Riot Twin Enso.
I took Sett to "kit lock" - which is nearly all mechanics designed and validated + working with concept art and narrative to flesh out who this character is. It leaves out most of the hard work of implementing things correctly for the long term, working with production artists to get everything looking cool and readable in game, plus numbers tuning/balancing and getting each detail to feel just right.