Victor Frankenstein describes his best friend, Henry Clerval, as a "boy of singular talent and fancy." He wrote a fairy tale at the tender age of nine, and he read voraciously. He would compose plays with various popular characters from his favorite stories for Victor, Elizabeth, and himself to act out. In other words, Henry's strengths were his imagination, his writing ability, and his ability to truthfully depict human lives in his own creative works. He also, as a young man, nurses Victor back to health when Victor becomes very ill after the creation of his monster. Henry was nurturing and gentle and kind.
In terms of his weaknesses, he is, perhaps, too trusting. He does not question his friend's integrity or intentions even when there is reason to do so; he only goes along with whatever Victor seems to need or want. Had he questioned Victor about his real motives in wanting to delay his marriage to Elizabeth, it is possible that Victor would have revealed to him that he'd created a monster and was now tasked with making its mate. Henry might have been able to protect himself or even distance himself from Victor (though it seems unlikely that he would). It is difficult to find a real weakness in Henry himself as it seems that his biggest mistake was his choice of friend.