The amazing super k-rad awesomeness. They don't get spells as normal, instead they summon a 7th party member, and can hold one monster type (including bosses) for each spell "level."
Mastery: Oath of Blood: Summoned monsters increased (Max 1221.5% -- so a level 40 Demon would become a level 488 nightmare.) Their abilities are adjusted and some become absolutely absurd. If I'm reading this right, monsters that have classes (like Samurai or Ninja), when modified by Oath of Blood, [i]gain their enhanced class abilities[/i].
Special EX Skill: Masutatterion (Summon... everything at once? Merge summons into one big summon? Not sure.) and "Oath of Soul" (turn summons into permanent PCs, making them into absurdly broken ... things)
Starting Class: B. They can pass on summons but why in the world would you -- without Oath of Blood, it's worthless. Ending Class: A. Holy god, summons are insane. And the Oath of Blood thing is nuts. No Class Change: C. They don't get much from innocent gear and really they're meant to be doing other things after summoning, without other spells they really are just sitting around wasting time.
Gothic added a lot more really good summons in the early/mid game. This is now an attractive class to use from the start instead of being an Alchemist/Bishop's retirement plan. Best to be 10 levels higher than your summon targets to ensure success. You will NEED a Brawler if you have this class, most of the good monsters have 100 or 98% resistance to summon contracts.
The PSP version has a bug where a Summoner can use Oath of Soul on summons from ANOTHER character. This was fixed in the 3DS. Oath of Soul party members are much better in Gothic than they were in Original, mostly due to them not getting Innocent Gear not being a huge disaster anymore.