I just got my first legendary card, Keeper of Souls, Yay!
How can I make best use of him?
Other than Keeper i have no legendarys, and very few epics:
Faunessa
Luther
Sparkgrinder
Swamplash
Some ultra rares. And most of the commons/rares.
I seem to be lacking purple mana producers, like Wight and Giant Spider. Are there other ways to get him going?
If you have no purple spammers, how about Acolyte for spamming brown? And back him up with Valkyrie (to max his blue quickly AND get extra souls). If you ever get the Green Slime, he’s the best option for purple spamming, BTW. But if you have Acolyte and Valkyrie, use those, and then add in a decent green troop to round out your colors, preferably something beefy to put in front, or failing that a simple Goblin will at least fill quickly and give you some extra damage. Just make sure whatever it is doesn’t seriously hamper your ability to power the Valkyrie or the Acolyte.
Also, I’m turning way green over here; level 267 and still no Keeper of Souls. You lucky dog. 
Almost any Mana spammer works well with Keeper’s Ability. Once you get him going, he can clear house.
A Troop that increases Attack also pairs well with Keeper.
Agreed, Green slime works extremely well with keeper of souls.
Completing kingdom quests will net you a bit more epics. A couple that will be great to finish would be Wild Plains and Pans Vale. This will give you Sunweaver and Sylvasi. Sylvasi works wonders with Keeper since he steals attack while converting yellow gems to purple. Sunweaver will boost Sylvasi all around. Throw in a Skeleton, and you have an unbeatable team.
Warning edit: Completing Pans Vale will not give Sylvasi. Leaving this up in case you find one in keys, since it is a nice team.
Er, not unless they changed it since I played through Pan’s Vale; you get Elwyn for completing the quests rather than Sylvasi, and Elwyn isn’t nearly as useful for pairing with the Keeper.
You are correct.
Try Skeleton, Kos and some support that creates purples, say slime.