I think Frederick is meant to have brilliant wit, which was "
"bewitching" to Anne but had the opposite effect on Lady Russell. However it is only described by the narrator in this para -- chap 4 --
"he was confident that he should soon be rich; full of life and ardour, he knew that he should soon have a ship, and soon be on a station that would lead to every thing he wanted. He had always been lucky; he knew he should be so still. Such confidence, powerful in its own warmth, and bewitching in the wit which often expressed it, must have been enough for Anne; but Lady Russell saw it very differently. His sanguine temper, and fearlessness of mind, operated very differently on her. She saw in it but an aggravation of the evil. It only added a dangerous character to himself. He was brilliant, he was headstrong. Lady Russell had little taste for wit, and of any thing approaching to imprudence a horror."