Jonathan is the first character we meet in the novel, but he's hardly the most interesting. (Want proof? Keanu Reeves played him in Bram Stoker's Dracula. Okay: That's not totally fair... but Keanu Reeves never changes.)
Whenever he shares a scene with another character, he tends to be upstaged. Dr. Seward describes him as a "quiet, business-like gentleman," "uncommonly clever," and "full of energy" (17.41), but that's about all we hear about the young lawyer.
The other men often feel sorry for Jonathan because his wife, Mina, is one of Dracula's victims. And while Jonathan is brave enough to be the first to jump out at make a stab at Dracula, he's also the least experienced of the men at fighting or hunting. After all, the guy is a lawyer—he's spent most of his life studying and working various desk jobs. He doesn't have Quincey's experience hunting buffalo on the prairie in America, or Arthur's experience hunting foxes on the English countryside, or even Dr. Seward's experience wrestling with homicidal maniacs in an asylum.
He speaks several foreign languages and can tell you more than you wanted to know about the English legal system, but that's about it. Overall, he's pretty ordinary... just your average middle-class English guy. Brave when he has to be, but otherwise pretty quiet. Maybe Stoker wanted to have an average character to balance out the eccentric, extraordinary characters like Van Helsing.