Because of your existence, I can be myself. (otherwise my life would be meaningless) キミがいてボクで If I didn't exist, you would also have a problem, wouldn't you? キミだって(ボクが)居なきゃ困るでしょ？

なんだかんだ言ったってボクら Eventually, we are 月と太陽みたいな like the moon and the sun. I guess it’s quite difficult to translate this part into English↓ キミがいて ボクで キミだって居なきゃ困るでしょ？ I believe the part is a contraction for: キミがいて ボク(がいて) キミだって (ボクが)居なきゃ困るでしょ？ I checked the whole lyrics of this song. “キミがいて” means almost the same as “キミがいる” or “キミはいる” (means like “You are here” or “You exist”) “て” for “がいて” makes the phrase related to next one. Sounds like “And~” or “Then”. So, It originally means like “You are here, and me as well”. “だって” for “キミだって” means like “you too” or “So do you”. However, it sounds a bit stronger than “キミも”. (Although these mean almost the same) I’m not sure if this is correct grammar, but it sounds like “I bet you’re stuck if you’re not with me, aren't you?” To put it another way, “You can’t live anymore without me like I can’t without you, right?" I hope my explanation is easy to understand:) Keep going!

- キミがいてボクで ＝　キミ＋が＋い＋て＋ボク＋で - i believe it's 「キミがいてボク」+で 　　　　　　　　＝　「"you are, then I am" で」 - で＝「に」＋「て」(the sound of ’NITE' got broken into that of 'DE') - 「に」means a target place, "where you are/it is/ do to,at", - 「て」adds 'a sense of perfection, in terms of aspect' - 「で（にて）」means a field, "where you do", e,g, 「海に行く」go to the beach (sea) 「海で遊ぶ」play in the beach +++ 「キミがいてボク」+で = *(in that 'キミがいてボク'）