The 19-year-old singer admits the 'x factor' chief tries to keep her and fellow panellists L.A. Reid and Britney Spears in line during filming of the talent search contest, but she isn't afraid to stand up to his bossiness.
She said: ''Simon's great, but he nags on me so much that I don't know if I can take it any longer.
''I'm a brat. I'm annoying. But I give it back to him.''
While Demi has clashed with Simon, she has made a firm friend in L.A., who says they will be best friends forever (BFFs).
He told 'Access Hollywood': ''Britney is such a massive star that it's slightly intimated. She's really sweet, just really loving and charming, but she's intimating because she's such a Big Star.
[Demi] is like my BBF for life. BFFL. She's really too good to be so young.''
As a judge, Demi admits she is very honest and sometimes finds it hard because she knows her opinion will upset the hopefuls.
She said: ''It's really hard for me to smile if they're performing and they're horrible.
''I have to be super super honest and sometimes I feel bad about it.''
Meanwhile, Britney thinks she is ''fair'' to contestants and tries to be honest without shattering their dreams.
She said: ''I think I'm a fair judge. Sometimes it's hard because I know what they've been through and being so young and [wanting to] achieving their dreams.
''It's such a subtle and delicate situation when they're that young so you just have to use constructive criticism and be there for them and just show them support.''