The key to making sense of how genuine Doppio's personality is may ultimately lie in the objective truth of what he does for a living. He may not know he shares his body with Diavolo, he may not remember his past, and he may, in fact, have a genuinely nice disposition - but he is the direct underboss to a drug lord who rules over Italy's underworld, and not only is he aware of that, he's fiercely loyal to him. In that sense, the assessment that he is a "walking contradiction" may actually be a very apt one: sweet and cowardly as Doppio may be, the Boss's orders take priority over all else; he will fight to the end for his boss, and he will be kind only as far as it doesn't interfere with his mission.
All of this is because Doppio and Diavolo are deeply, (almost) inseparably linked, and that's precisely what makes it so hard to pin down where one ends and the other begins. Again, though, a closer look at canon may make things a little clearer: both of them have anger to spare, but the way they express it is different. While Diavolo tends to be more composed and purposeful - unlike many other stands in the series and their infamous punch rushes, his King Crimson seems to prefer delivering a single, fatal blow - Doppio is erratic and unfocused. Diavolo has to stop him from gouging a cab driver's eye out; Diavolo asks him to calm down when Doppio's temper explodes in his fight with Risotto. While Doppio's mood swings do seem to be closely tied to dissociative states, or simply to Diavolo taking over, the way his anger is expressed is still uniquely Doppio's, and that must mean the anger itself is his as well.
In summation: Doppio and Diavolo's joint existence muddles things up, as they seem mutually protective of each other - fitting for a bizarre Jojo-flavored DID - and as such their behaviors can seem tangled up. However, looking at Doppio's actions in canon and at ALL the ways he seems to differ from Diavolo, rather than just the obvious ones, it can be surmised that the naive, boyish parts of Doppio's personality can and DO currently coexist with his more violent tendencies, and both are independently true, no matter what the situation may have been in the more distant past or "who" the original personality really was.
ADDENDUM
All of this is because Doppio and Diavolo are deeply, (almost) inseparably linked, and that's precisely what makes it so hard to pin down where one ends and the other begins. Again, though, a closer look at canon may make things a little clearer: both of them have anger to spare, but the way they express it is different. While Diavolo tends to be more composed and purposeful - unlike many other stands in the series and their infamous punch rushes, his King Crimson seems to prefer delivering a single, fatal blow - Doppio is erratic and unfocused. Diavolo has to stop him from gouging a cab driver's eye out; Diavolo asks him to calm down when Doppio's temper explodes in his fight with Risotto. While Doppio's mood swings do seem to be closely tied to dissociative states, or simply to Diavolo taking over, the way his anger is expressed is still uniquely Doppio's, and that must mean the anger itself is his as well.
In summation: Doppio and Diavolo's joint existence muddles things up, as they seem mutually protective of each other - fitting for a bizarre Jojo-flavored DID - and as such their behaviors can seem tangled up. However, looking at Doppio's actions in canon and at ALL the ways he seems to differ from Diavolo, rather than just the obvious ones, it can be surmised that the naive, boyish parts of Doppio's personality can and DO currently coexist with his more violent tendencies, and both are independently true, no matter what the situation may have been in the more distant past or "who" the original personality really was.