At the end of the play, an exchange between Bernard and Hannah sums up Ezra Chater's character pretty well:
BERNARD: (Wildly) Ezra wasn't a botanist! He was a poet!
HANNAH: He was not much of either, but he was both. (2.7)
Chater bounces from occupation to occupation, and at times from thought to thought, with about as much awareness as a pool ball. When Septimus wants to convince him that his intentions towards Chater's wife were, if not honorable, at least beneficial to Ezra himself, all he needs to do is to give Chater a push in the right direction:
CHATER: (Shrewdly) Did Mrs. Chater know of this before she – before you –
SEPTIMUS: I think she very likely did.
CHATER: (Triumphantly) There is nothing that woman would not do for me! (1.1)
Aside from the whole moral black hole of Chater thinking, the ease with which Septimus manipulates Chater into not just forgiving him, but also thinking he's a great guy, speaks to Chater's lack of a strong will. Wherever the other characters push him in the play, whether it's to fighting a duel or sailing to Martinique, Chater barely resists.
So he's weak...so what? Well, there's this thing called fate vs. free will, you may have heard of it. One of the central questions of the play is this: to what extent are human beings free agents to do what they please? When are they just following natural laws set down before they were born? Chater is Exhibit A of how a person can think they have free will when really, he's just a reed bending before any manipulative wind that blows his way.