Learn Hardcore Japanese: motiron asita mo kuru yo. - Of course I’ll come tomorrow too.

ね: seeks agreement/confirmation (“right?”).
よね: assertion + seeking agreement (“I will, right?” / “You know I will, right?”). No よ = more neutral statement.

It adds the meaning “too/also/as well” to the time word. So 明日も means “tomorrow as well,” implying there’s at least one other relevant time (e.g., today, yesterday, or another day) when the coming also happens. Without も (just 明日来る), you simply state “(I) will come tomorrow,” with no additive nuance.

Where is the subject “I”? Why is it missing?

Japanese often omits subjects when they’re clear from context. Here, the speaker is talking about their own plans, so “I” is understood. You can add it for emphasis or clarity: 私は（俺は／私も）明日も来るよ。 Overusing explicit subjects can sound unnatural, so only add them when needed.

Why is the plain non-past 来る used to mean “will come”?

Japanese doesn’t have a dedicated future tense. The non-past form covers both present and future, and time expressions like 明日 supply the future meaning. So 来る here naturally reads as “will come.” To express intention more strongly, you could use 来よう (volitional) or 来るつもりだ／来る予定だ.

Can I say 来ます instead of 来る? What changes?

Yes. 来ます is the polite form. Choices:

Casual: 明日も来るよ。
Polite: 明日も来ます。 (You can also say 明日も来ますよ。) Choose based on relationship and context. Mixing plain and polite in the same sentence is usually avoided.