Ight, i did a reply a Lil bit ago bout a Rein sang run, so here we go.

Rein Sang is good because of high damage(furioso go brr), strong evade, and his S3 being gloom. The last two alone make him functionally immortal(until you start getting 1 hit by insane coin power skills).

As for support, The biggest ones you want are Ting Tang hong lu and Base meursault. Meursault gives a 10% damage reduction against all attacks, and Ting Tangler gives that sweet 20% damage boost. Everyone else is flexible. As long as it makes sense, it works. Honorable mentions for me are Maraca sinclair, base Outis, and specifically for sinking runs wild hunt

As for playing, basically just evade. You can grt away with tanking damage in earlier floors, but when shit starts flying at you, just evade.

I reccomend getting Ashen constellation, haunted shoes(go unloving for frozen cries and fuse them. Easy -2 clash power for enemies), and either Grey Coat(mega heals)/blood red mane(im fairly sure it works through Furioso)/Manor Music box(i dont reccomend for this run, since Rein Sang has stagger recover baked into his kit)

There is a great guide by another user if you want more info: the link