what is the role of "立ち" in below sentence?

Question

I know the total meaning of this sentence:

黒田君達 特Sが上に立ち望月君達が それを下から支える役回りというわけなんです。

but I want to know the role of 立ち in the sentence. Is it works as an infinitive verb?

Answer 1

This 立ち is the masu-stem of 立つ used as 中止法. It's a bookish or more formal way of continuing a sentence used instead of a te-form.

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黒田君達特Sが上に立ち、望月君達がそれを下から支える役回りというわけなんです。
= 黒田君達特Sが上に立って、望月君達がそれを下から支える役回りというわけなんです。

Kuroda and other members of the 特S (Team) will stand at the top, and Mochizuki and his team will play a role of supporting them from below.

This 上 ("high position") refers to either a physically high position (e.g., roof) or a socially high position (leader/commander). See the previous context.