けなげな describes someone’s sincere, selfless effort, often with a touching sense of vulnerability or courage. In the song, it refers to the heartfelt feelings written in the other person’s awkward letters, emphasizing how their quiet devotion repeatedly saves and supports the singer.
"NEO VENUS" is a love song dressed up as a prayer to a star. The singer keeps calling out to VENUS and asking it for reassurance: 「僕の翼は今もまだ白く輝いてるか?」 (are my wings still shining white, even now?) and 「間違ってないか?」 (am I not going the wrong way?). It's a big, slightly dramatic way of asking "am I still the person I meant to be?", which is very Janne Da Arc.
The twist comes after the pause: the one who gave him back his 「散らかったパズルの様な心」 (heart like a scattered puzzle) and his half-lost dreams was 「あなたでした。」 (it was you). So VENUS and "you" start to blur together. In the second verse he admits that 「かしこく生きる事にも少し無理があるみたい」 (living cleverly seems a bit beyond me), and the thing that saved him again and again was a voice calling his name, plus a letter in 「飾らない言葉と不器用な文字」 (plain words and clumsy handwriting). That detail is the sweetest part of the song.
The chorus pushes forward 「向い風の中でも」 (even into a headwind), promising 「あなたを離しはしない」 (I won't let go of you) and 「僕達だけの歴史をつくろう」 (let's make a history that's only ours). Learner tip: 「~だろう」 here is the singer's resolve, "I'll surely search for the answer", and 「離しはしない」 uses は to make "I will not let go" extra firm.