Informal:
* Mate: This is the most common and versatile replacement, often used among friends.
* Pal: Similar to "mate", but slightly less common.
* Chap: This is a bit more old-fashioned and often used humorously.
* Friend: Simple and straightforward, but slightly less informal than the others.
More Formal:
* Companion: A formal word for someone who accompanies you.
* Acquaintance: A person you know slightly.
The best choice will depend on the specific situation.
For example:
* "Hey mate, what's up?" (Informal, friendly)
* "I went to the pub with my pals." (Informal, group of friends)
* "He's a good chap." (Informal, humorous)
* "I'm going to the cinema with my friend." (Less informal, friend)
* "My companion and I enjoyed the concert." (Formal, companion)
* "He's just an acquaintance, not a close friend." (Formal, acquaintance)