General Synonyms:
* Companion
* Comrade
* Pal
* Buddy
* Associate
* Acquaintance (less close than friend)
* Confidant (someone you trust with secrets)
* Soulmate (a very close and intimate friend)
Informal Synonyms:
* Bestie
* Sidekick
* Bro/Sis
* Mate
More Formal Synonyms:
* Associate
* Colleagues (for friends in the workplace)
* Close acquaintance
* Fellow
* Intimate
Figurative Synonyms:
* Brother/Sister
* Family
The best synonym to use will depend on the specific situation and the level of closeness you want to express.