I don't feel like she's a side girl anymore (especially since 0.6) and I'd even say that she's one of the most important girl(could be even
the most) given that without her, Orion and the girls wouldn't have been able to recover the gems of doom (cf. The Heist and during the war server, Orion would have encounter Thanatos leading to a battle I'm sure he would have lost in his current state).
Caribdis said indirectly that Calypso won't be part of the Harem but she will have a special treatment:
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So yeah in terms of "sexy stuff" guess you could still consider her a "side girl" but storywise it is very unlikely and she will become even more important for the plot later on.