IMO Kellam should be High Tier. He is VERY helpful in the first chapters and he is decent/good to use in the endgame. He has good str, decent speed and great def, which is very rare in the beginning. He is also great for pair up, giving a huge boost to str and def that becomes better when you level up your support, saving your ass many times. He also has access to Pavise, one of the best abilities in the game! His only flaws are terrible mobility and res. So basically, you can put him against anything except mages and barbarians.