a guy

A "guy" is a man. It's a casual word.

You use this word instead of "man" when you're talking to your friends or in a casual situation like at a party. The person you're talking about can either be someone that you know, or someone who you don't know but don't have high respect for.

For a stranger who seems older and more respectable, "man" or even "gentleman" are more polite.

Note that you can use the plural form, "guys", when you're talking to a group of men or women:

Great job today, guys. Keep up the good work.

But "guy" (singular) always refers to a man.

(someone) is seeing (someone)

"Seeing" someone means dating them or in a romantic relationship with them. For example:

Are they seeing each other?

I was interested in her at the time, but she was already seeing someone.

You might not use "seeing ___" when talking about the past:

That was back when they were dating.

You also wouldn't use "seeing ___" to talk about a couple who has been together for a long time:

They've been together for, what, like three years now?

(someone) is shady

A "shady" person is someone who seems bad, dangerous, or dishonest. For example, if you are suspicious that someone sells drugs or cheats on his girlfriend, you can call him "shady". 

If you know that a person does bad things, don't use "shady". Only use it if you think they do bad stuff.

You can also say that a person "is acting shady":

What's up with you? You've been acting awfully shady lately.

a (adjective) character

The word "character" marks a person who's not exactly normal. They seem more like a character in a book or movie than a normal person. Here are some common expressions that use "character" this way:

He's a real character. (He's really unusual.)

She's quite a colorful character. (She's loud, talkative, opinionated.)

I saw a suspicious character lurking outside your door.