For most of the book, Colonel Morden is like a ghost in a scary movie. You know the ghost is there because everyone can't stop talking about it, but you start to think it's just made up until BAM! It pops up and scares the bejeebies out of everyone. Okay, so Colonel Morden isn't scary (unless he's wielding a sword in duel-mode). It's more like he's the dude Clarissa thinks is going to save the day, but his timing's just a little off.
In some ways, Morden is like the sibling Clarissa never had. Once he finally arrives on the scene, he's willing to go to bat for Clarissa in a way the rest of her family never are. After Belford lets him know the whole situation, Morden exclaims "bitterly" against Lovelace and Clarissa's "implacable relations" (476.3). The guy definitely beats himself up over arriving too late to prevent the whole scandal.
But oh, man, does he make up for it. Morden shows his true colors when he tracks down the fleeing Lovelace. It's a major move, so let's repeat that one more time: Morden chases Lovelace internationally in order to bring him to justice. Oh, and after mortally wounding Lovelace, Morden "was as cool as if nothing so extraordinary had happened, assisting the surgeons, though his own wound bled too much" (537.8). We have it from Samuel Richardson's own hand: Morden is a cool dude.