What do I fodder him for?

It depends what you have already and close to building. Lyns bow is already pretty good, and if bridelia already has brave bow+ then building 2 brave bow users might be a bit redundant imo, though still very useful. If you're building a flier team or just need a green nuke, Cherche is awesome with DB.

If you don't have a brave bow user, then fodder him for that, unless you wanna build another. Other than that DB

You definitely shouldn't use him just for Death Blow because he already has DB3 at 4*. And honestly I don't think the difference between Quick Riposte 2 and 3 is that relevant, so I'd go with Brave Bow+ for sure. I guess if you're going to give it to B!Lyn or Bridelia is your choice, but I can say I gave one to my Lyn and it improved het a lot (though I don't use it in arena for score reasons). Also, B!Lyn already have Swift Sparrow as her A skill while you could give Death Blow 3 to Bridelia if you sacrifice a 4* Effie, Hawkeye or Ursula to give Death Blow 2 to her before you feed Klein's Brave Bow+ and Death Blow 3. But really between the 2 archers I think it comes down to your preference.

I hope i'm not too late to answer your question. If i were you I'd inherit evrey possible skill from that klein. How ? let's imagine it's for your gaius : you'll want to inherit brave bow, brave bow+, death blow 1 2 and 3, QR 1 2 and 3. For that, you'd first inherit QR 1 and 2 (and probably swap) from a subaki, a brave bow (and other skills) from gordin, and death blow 1 and 2 from ursula or hawkeye. Then you can inherit Brave bow +, death blow 3 AND QR 3 from klein. Even if all those skills are not handy right now, you'll be happy to have them when needed.