Like Arthur, Tristram is another knightly figure pulled from a long, pre-existing tradition of British legend: Tristan & Iseult, a pair of star-crossed lovers that could give even Romeo and Juliet a run for their tear-jerking money.
But unlike Arthur, Tristram doesn't play a very big role in the poem, really just making a cameo in the beginning of Book 6. We meet Tristram pre-Iseult and pre-knighthood, when he's just a promising young lad who killed a really mean knight.
While Calidore is initially wary of Tristram's knight-killing ways, he's impressed when Tristram explains the whole story and admits that Tristram had pretty good grounds for doing what he did. Recognizing that Tristram has potential-hero written all over him, Calidore knights him and sends him on his way.
Tristram demonstrates that knighthood isn't an absolute quality since some knight are un-knightly (boo) while some non-knights are very knightly (huzzah!). Calidore's encounter with Tristram is also a nice way for Spenser to link his poem to this venerated, legendary tradition of English literature, and even make his world—Faerie land—the origin point of that legendary tradition. Famous knight Sir Tristram? Oh yeah, he got his start in a little place called Faerie…