So I just got Waver...

Level him up anyways, since imo Waver is a better generalist support than Merlin. It will be quite some time until Merlin arrives anyway.

Although you should prioritise on ascending him first to get all 3 skills first.

Merlin would be better than Waver if you already have a Buster and crit servant like Jalter, Gilgamesh, Ozymandias, etc. However, as someone else already said, Waver is a generalist support and would arguably serve as well as, if not better than, Merlin if you lack buster crit servants. Besides, you've already rolled Waver. I'm not trying to curse you or anything, but there's no guarantee you can pull Merlin from the gacha in December either unless you got 1k+ quartz.

First waver, try to max him. Before Merlin's arrival we'll have both christmas events, so you can get enough skill gems for Merlin.

Waver - goes with anyone, including memelin.

Merlin - can go with anyone, but only really shines in a bust crit team.

They can both cover each others 'weaknesses'. In general use Merlin if you can kill something in 1 turn and Waver for anything else assuming you don't have both. Merlin needs to keep up his np to be at his best (not hard to do), but you'll want to time your skill use. Waver has utility skills with effects lasting 3 turns you can shoot off at anytime with almost no drawback.

Welcome to the club, raise him good enough and you will have so much benefit such as 3 turn farm and lightning fast boss kill or immortal Stall team ( there's more of course ). Other Cases if you aren't getting Merlin, Waver is your best choice.

Waver = OG FGO legal cheat Code
Merlin = Yet another FGO legal cheat Code