According to some stories within the Transformers canon, each of the characters represented by the original 1984 Transformers toys is a legend. However, that assessment is sometimes hard to reconcile with the reality that some of these characters received little to no characterization at all, and comparatively few appearances in the fiction in which any personality might be expressed. Trailbreaker is one of these ciphers.
Even if you were to ask a hard-core Transformers fan, pretty much the only thing they'd be likely to be able to tell you about Trailbreaker without looking him up is that he's "the force-field guy." With few exceptions, if Trailbreaker does show up in a story, it's because the script calls for a force-field to be used in that situation, and Trailbreaker's the go-to guy for force-fields. When this new toy (now called "Trailcutter" for trademark reasons) was released just a couple of months ago, the included pack-in comic book even made a point to talk about the fact that so little is known about Trailbre...uh, Trailcutter besides the fact that he can do force-fields.
The original 1984 toy's Tech Specs bio did make an attempt to tell us more about the character (and some of these traits do survive into the above-mentioned comic story, if arguably for the first time ever). He demonstrates a public face of humor and optimism, apparently to cover up a low sense of self-worth, being the Autobot that (of that original group, at least) uses up fuel more quickly than anyone else. Given that the need for fuel was perhaps the single most consistent plot point of the early Transformers stories, it perhaps makes sense that being so much of a gas-guzzler would cause one to consider oneself a liability, but I can't recall any story of the 1980's that made note of this point (readers are invited to cite examples in the comments if they know of any).
Having established what little there is to say about Trailbreaker's personality, I did want to point out an oddity about this recent toy. The accessory they gave Trailbre... uh, Trailcutter (that name's going to take some getting used to) is a shield. I haven't seen anyone else comment on this yet, but this seems a singularly odd choice of weapon for the character whose one well-known character trait is that he creates force-fields. Why would such a character need a physical shield? (I've got the toy holding it upside down, compared to the character art and the instructions, which probably highlights the "shield-ness" of the item even more than intended, but even with those gun barrels pointing the right way, there's little denying that this thing is a shield) Perhaps the inclusion was meant to be symbolic or something, but it just strikes me as weird.