Ralph Marvell comes across as way too trusting, and it is his trusting nature that is ultimately responsible for his losing his money, his son, and, eventually, his life.
Marvell realizes that, in marrying Undine, he's ended up married to a woman so obsessed with money that she's prepared to do whatever it takes to get it. But such is his character that he's still prepared to work hard to provide Undine with the money that she craves, even though it's never enough.
Had Ralph been a stronger character, he would not have succumbed to Undine's shameless attempts at blackmail concerning the custody of their son Paul. Given the social mores of the day, it wouldn't have been too difficult for Ralph to portray Undine as an unfit mother on account of her numerous extramarital dalliances.
But being the all-around nice guy that he is, Ralph borrows money to give to Undine in the hope that it will be enough. It isn't, of course, and so a distraught Ralph kills himself, losing his beloved son in the process.