Kassius fills a few rolls. Between himself and Kibeti he can do a fair bit of damage dealing, both melee and ranged. He has a fair bit of magic, but it will be primarily for self-buffing and out of combat utility. He also can help Logan with scouting and sneaking, particularly in wilderness settings, whereas I assume Logan will be better in urban areas.
In terms of the party roles you discussed, there's basically two aspects to consider, spell selection and feats. If you're going the Ray/ranged touch, Point Blank shot and Precise shot are fairly important, although you can survive without them. Summoner is significantly boosted by Augment Summoning, which requires Spell Focus (conjuration). Going pure blasting usually requires significant optimization to be competitive, I recommend against it, although picking up fireball or something like it for dealing with the occasional crowd of mooks is definitely useful.
If it were me, I'd probably taking the Augment summoning line of feats to have a bit of a focus, but generally pick spells in both the buff, blast, and debuff lines, to maximize my options. The strength of spellcasters they can have so many pretty good options something is bound to be useful. Magic resistant foes: buff or summon. Lots of mooks: blast. Single tough enemy with good saves: debuffs with no save spells.
If you're looking for suggestions, on the pathfinder forums, there are various guides. The guide to wizards here:
http://www.d20pfsrd.com/extras.../treatmonks-lab/test
is quite good, and the spell list section will be mostly equally applicable to you, although it hasn't been updated in several years, so it doesn't have the spells from newer books. Might be worth a read-through though if you're interested.