Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) It was on your bed, the box. I just thought I'd try one. Or twenty? I can't stop thinking about her, Harry. Honestly, you know, I reckon she was starting to annoy you. She could never annoy me. I think I love her. Well, brilliant. Do you think (..)2009 Harry Potter and the Half-Blood Prince
(..) doing it? Half the alley's closed down. Fred reckons people need a laugh these days. I reckon he's right. Oh, no. Everyone got their wands from Ollivanders. Harry... Is it me (..)2009 Harry Potter and the Half-Blood Prince