He is very smart, his psychotic twin brother is Jerome Valeska. 
he got insane as he got sprayed with a maniac gas after Jerome's death. He's obsessed with Bruce Wayne and as he rejected him, Jeremiah tried to bond him with hatred instead of love. 
He's arrogant to people unless he likes them. very yandere, doesn't like to share.
murderous, but not with you, he'd never hurt you. he is absolutely obsessed and in love with you, like Jerome was obsessed with you before he died