Sincere: to be honest and trust-worthy.

I think she was being sincere, so I think we can trust her.

Sometimes, it's very difficult to know if someone is being sincere or is just a good liar.

(He made many promises, but I don't think he was being sincere, so we shouldn't depend on him.)

Naive: overly simple and lacking experience.

He wants to change the world, but he is very naive, so he needs someone to help him.

I think we can take advantage of him, because he is naive and impatient.

(She's still young and naive, so we can't give her too much responsibility without someone to guide her.)

Brilliant: highly intelligent, or exceptional in brightness, the opposite of dull.
明亮的才华横溢的 聪明的;         优越的;           亮度; 明亮                           迟钝的; 阴暗的

There is no doubt that he is a brilliant scientist, but he is a very poor manager.

It's amazing how such a brilliant man can be so stupid around women.

(He's brilliant but naive, so he needs someone with a lot of experience to help him.)

Cruel: mean, manipulative and hurtful, the opposite of kind.
残酷的 刻薄的 爱摆布人的            伤感情的;

He can be cruel when he doesn't like someone, so don't expect him to be nice to you.

I was shocked by how cruel he was when he fired her.

(Some people can be really cruel, so don't be naive or you can be hurt.)

Clever: innovative, creative, very smart.革新的; 创新的;

He found a clever solution that allowed us to reduce costs.聪明的，灵巧的

Advertisers are very clever at getting people to buy things they don't really need.

(They have developed a very clever way to increase the efficiency of their sales team.)效率，效能;

(He isn't as clever as he thinks he is, so we may be able to find a weakness in his plan.)

(It appeared to be a clever idea at first, but it failed because it ignored some important details.)

Skeptical: needs proof or evidence to overcome doubts, the opposite of convinced.
怀疑性的，好怀疑的                                            怀疑，疑虑                     确信的; 深信的;

I'm sure they will be skeptical of our results, so we need to bring convincing evidence.

I'm skeptical that such a simple design can satisfy our requirements.

(Your ideas are impressive, but I'm still skeptical, so I need more evidence.)