Silhouette (シルエット)

Boundary crossing, look-back thing without

一線を越える ('to cross the line') is a fixed idiom for crossing into adulthood, breaking a taboo, or otherwise stepping past a known limit. The image fits Naruto Shippuden's transition from childhood arc to wartime arc.

We (topic) anything-and-everything still don't-know (literary)

知らぬ is the literary/classical negative of 知る ('to know') — equivalent to 知らない. Common in lyrics for the heavier register. Pairs idiomatically with 何もかも ('everything').

Don't-remember things also, a-lot existed probably

Anyone-and-everyone, silhouette

誰も彼も is an emphatic 'every single person' — stronger than 誰でも ('anyone'). Combined with シルエット ('silhouette'), the line frames everyone as outlines, no longer fully visible.

Cherish was-doing things, forgot-pretense did (it.is) (emphatic)

Verb-past + ふりをする ('to pretend that one did X / to act as if'): 忘れたふりをする ('pretend to have forgotten'), 知らないふりをする ('pretend not to know'), 寝たふり ('pretend to be asleep'). Standard conscious-deception phrase.

Set-go-with let's-remember, we (topic) anything-and-everything (object) wanted

せーの (often せーので or せーのっ) is what Japanese kids and adults yell to time a synchronized action — like 'one, two, three' before lifting something or jumping together. 欲しがる ('to want', 3rd person) is the third-person form of 欲しい — for describing others' desires.

Understand-am (quotative), notice-am (quotative)

Sentence-final って is a casual quotative — like saying '(I told you) I get it'. It can sound mildly defensive or insistent depending on intonation.

Clock's hand (topic), days (topic), stop

Anyone-and-everyone, silhouette (object) fearing, remain-over thing

Surely all-along not-change thing (subject) exist thing, taught-me-and-given if

教えてくれる ('to teach me / let me know') uses giving-verb くれる to mark the action as a kindness done for the speaker. 〜たなら ('if you had X') is a wistful past-conditional.

Want-to-cherish things holding, adult become (it.is)

大人になる ('to become an adult') is the standard Japanese phrase for 'growing up' — applied both to age (adult age) and maturity. The line is the song's resolution.

Any-kind time even, without-letting-go, let's-keep-protecting

Verb-stem + 続ける ('keep doing X'); 続けよう is volitional ('let's keep'). Productive: 信じ続けよう ('let's keep believing'), 探し続けよう ('let's keep searching').

If-(we)-do-that, when-day (uncertain)

そしたら is the casual contraction of そうしたら ('if (we) do that'). いつの日か ('someday', literally 'on which day, who knows') is more poetic than just いつか.