Like Launcelot, Trystram is an honorable, loyal vassal to his king (King Mark) who happens to be involved in a passionate love affair with that king's wife. It's a sticky situation, to say the least. But Trystram is also the best knight in his kingdom, Cornwall, and, when he travels to Arthur's kingdom, the only knight who can even battle him to a draw is – you guessed it – Launcelot.
Unlike Launcelot, though, Trystram is a bit more of a renaissance man, excelling not only on the battlefield, but also in academics, hunting, hawking, and harping. In fact, the only reason he meets and falls in love with the great love of his life, Isode, is because he's her harp instructor.
Another important difference between Trystram and Launcelot is that Trystram's king and uncle, Mark, is not an honorable king. In fact, he's pretty much Arthur's opposite – a man who murders his own brother and tries to murder Trystram. Yet despite this, Trystram behaves with nothing but loyalty toward Mark, serving as his champion on multiple occasions.
At one point, he even engages in one-on-one combat with the chief of a Sessoyne army that threatens the kingdom when he's still recovering from terrible injuries, just because Mark asks him to. There is a breaking point, though. And when Trystram finally decides he's had it with Mark, he travels to Camelot, where he hangs with Launcelot's crowd and proves himself on the battlefield.
Trystram's relationship with his lady-love, Isode, is a bit more ambiguous than Launcelot's relationship to Gwenyvere. For one thing, Trystram falls in love with Isode when he drinks a love-potion, which seems to us like a little bit of a cop-out: who's to say these two would have shared an undying love were they not magically influenced? At one point, Trystram actually marries another woman (also named Isode, strangely enough).
Although he remembers his first love at the last minute and declines to ever sleep with his actual wife, we can't help but wonder where his "undying" love for Isode was when the dowry and influence of his wife's powerful family seemed too good a deal to pass up. Just as Mark is Arthur's mirror image in a kingdom slightly less perfect than Camelot, Trystram is a slightly-less perfect version of Launcelot when it comes to his loyalty in love.