It's hard to talk about Kwame without also discussing Hildegard, as both have identical special abilities and play a similar role. Kwame has a big edge in Speech Defence, is even worse at fighting and is stuck targeting only adjacent targets... so advantage Hildegard!
-Unlike Hildegard, you never get a double attack, and because he can't fight at range he's often exposed on the front line.
-Kwame's captain ability- +1 Study for each Friendly win- is good, but generally less than the Prestige/Science bump Hildegard gives you.
-He is a better healer, although his Excite ability also increases an enemy's Speech, making it more dangerous to attack with than Hildegard's Impress!
-You can improve his Cheer ability, but it never gains the battle-tipping area effect of Emilia or Phillipe's Tranquility.

So... not so great. Not terrible, and a solid support character, but his captain benefit is minor unless you're determined to go Friendly/Science, and there's nothing he can do that other characters can't do better. Still, some strategy ideas:

-Just like Hildegard, try for the Viking Drinking Horn and as usual, any Follower who increases your bonuses for Friendly wins. Weirdly, Hannah Haas works better with Kwame than she does with Hildegard.
-Keep Kwame back to heal and power up Emilia, Pedrinho, Yvonne, Kiwi, Anna... anyone with area effect speech abilities. His +25% Speech boost can make a big difference on the abilities he DOESN'T have.
-Kwame's Dance is slightly more useful than Hildegard's because of his higher Speech Defence. If you are about to lose someone he can swoop in to take speech hits. Even with a Decent Momento he becomes hard remove with Speech.
-Your attack is bad. Your Terrify is strong but inaccurate. Your Excite makes enemy Speech stronger. Unless you can take out a target with one shot, hang back and Excite allies, especially those who Impress.
-Because you're getting a Science bump, consider hitting the Caribbean Island first- usually the best first choice anyway. Your skills won't help with much, but since you're stacking Study anyway, it's probably a better start than the Study and Treasure-scarce Hungarian Fortress or Mali.