One particular adjective that can be used to describe Harker would be determined. He is determined to escape the Count's castle. He is determined to be reunited with Mina. He is also determined the kill the Count once Mina has been bitten. In these instances, the idea of being "determined" is something that can be used to describe Harker. Another adjective that can describe Harker is dutiful. He is bound by his duty of service to the Count. He is also bound by the dutiful loyalty that he feels towards Mina. In wishing to kill Dracula, Harker is carrying out the duty that is his responsibility as a husband. Harker can be seen as dutiful in how he carries himself. It is also in this manner where Harker can be seen as conventional. He does what is expected of him, and what is required. Harker does not seem to possess anything in way of intense and profound emotions. He is bound by the conventional sense of duty and honor that is a part of what it means to be a man during the time period. It is in this realm where I think that Harker can be described as "conventional."
In response to Ashley's answers, I agree with all of them except that I would point out that Jonathon Harker is not a static character in the story. So while he starts out conventional, as Ashley put it, to the point where he endangers himself to avoid offending, which I would describe as conformist, it becomes trivial after his encounter with Dracula. That is where his determination begins, as well as his passion, both in hatred and in duty. He becomes fearless, as he goes after the Count driven by that passion, even though the Count has proven himself more powerful. Harker also becomes resilient, as he had been tortured and put in nearly impossible situations, both physically and mentally, that he pushes himself through and survives.