Give some English expressions of different types of friends.
1. next friend: a person admitted to appointed by a court to act for the benefit of a person (as an infant) lacking full legal capacity 诉讼代理人
2. mutual friend: a person who is the friend of two people who may or may not know each other 共同的朋友
3. bosom friend: a friend that you like a lot and have a very close relationship with 知己
4. pen friend: someone you exchange letters with as a hobby but whom you usually not meet 笔友
5. fair-weather friend: someone who is good friend when it is easy for them to be one and who stops when you are having problems 只可工安乐而难共患难的朋友