The luck you have to get Sophie makes me somewhat green with envy.

But oh well.

In the case of Rainbow fairy sinks, Odette is always one to work on since the more closer she is to 16/16 the more potent she is against liches for any map. Having Sophie more or less can make your day (assuming you use her often enough in any particular map to justify the rainbow sinking), since the longer her skill is active the more damage she can do. Maribel's skill is a decent one to boost since in her case you want it to have a longer uptime, after all the best Maribel is one with her skill active. But compared to the previous two, Maribel looses out on priority. Assuming they do see use.

In Lyla's case, I'd only sink rainbows into her if you had no other units that can deal with liches. Considering 0 defense means nothing to magic damage dealers if she is one of your planned anti-lich units I'd say go ahead, but I'd still recommend sinking rainbows into Odette so that her skill is potent enough to help in dealing with the likes of those liches.