They're the primary protagonist faction, but several other factions have joined their cause as well (usually by sending representatives) to help them in the fight against the Reunion Movement. This may count as The Alliance, and is also the reason why you could justify and explain how the player is able to recruit operators from all sorts of allied factions as a gacha mechanic. The ones listed below are some incidents in the main story where Rhodes Island calls for help.
1. The negotiation with Lungmen happens in Chapter 2. Under these terms, Wei Yenwu allows Rhodes Island to use the ports of their city, but with conditions applied. The first one is for Rhodes Island to find any Reunion threats and any unregistered infected who might have infiltrated Lungmen. R.I. members also had to operate under the orders of the L.G.D.
2. When it comes to additional security expertise, they rely on Blacksteel. For example, Franka and Liskarm assist them in the search-and-rescue operation for Misha on Chapter 2.
3. With Penguin Logistics, it's mostly contractual, as Exusiai explains in the second and third chapters. They could leave the scene when the objectives are completed, but they can still be called anytime as Rhodes Island wishes.
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