* "Friend" - This rhymes with "send" but doesn't quite capture the same meaning as a "close companion." It's more general.
* "Blend" - This rhymes with "send" but doesn't have a strong connection to the idea of companionship.
* "Attend" - This doesn't rhyme exactly, but it could be used metaphorically. Someone might "attend" to a close companion.
Ultimately, there's no single word that perfectly rhymes with "send" and captures the idea of a close companion.