Sailor ?

She could be the sailor of light, because rainbows are made of refracted light. She could also have something related to a prism for a weapon, because when you shine white (normal light) through a prism, the prism refracts it and the light that comes out the other side has been seperated into the different colors that make up white light, creating a "rainbow." A prism scepter may be a good weapon, but I don't know what you would call it. Since the other senshi tend to be capable of multiple attacks you could some sort of mist or shower attack, because in nature the light that creates rainbows is refracted by water in the atmosphere. Or you could do a combination of a water and light attack: first the enemy gets hit by the water element then a light that will create either a single rainbow or multiple rainbows that will hit the enemy. Light themed attacks are good too, for example you could give her a "blinding light" attack; you could also use properties of light, like how light can be directed using mirror by giving her an attack that bounces a rainbow stream off mirrors, which appear around the enemy, until in finally hits the enemy. You could also give her a move that confuses and/or distracts opponents (giant rainbow, an illusion that conceals the real attack, a hypnotic effect, etc.), because rainbows are pretty and eye catching.

Also I like the names the others came up with. I like the suggestion to use Iris as the senshi name ciel as her human name. It could go either way, but because Iris was a goddess it makes sense that there would be a sailor Iris and that way her speech could be "I'm sailor Iris. Guardian of light and sworn to ward off darkness. And that means you. In the name of Iris I shall punish you!"

It also sets up a few potential back stories: you could have her be a servant of the Greek goddess, you could have her be the daughter/descendant of the goddess, you could have her belong to a group that still worships the Greek goddess in modern and have her be the chosen warrior of Iris, you could have her be from ancient Greece (whether that is before a rebirth or if she is still alive), you could make her the daughter of sailor sun and someone from a water themed planet, you could have her be from a secret relem (like elsyium), where they worship Iris (not necessarily as a Greek goddess, you could have Iris be a goddess that existeted outside of Ancient Greece), you could have her come from a planet where they worship Iris, or a combination of these. I have the most ideas about the plant though. You could have the planet named Iris after their goddess who may done something for the people of the planet (saved them from destruction, saved them famine, gave the the gift of knowledge, gave them her protection, gave them the gift of light, founded the planet, etc. something fitting and legend worthy). You could have the girl be the princess of the planet; you could even have the royal family be descendants if the goddess. It could be a tradition in the royal family to name people the French "arc of color" (like the Serenity thing in the moon kingdom), in honor of their goddess. That would fit the theme of the senshi being princesses of their planets.

You could have her protect sailor moon, have her protect someone else or have her be the one protected by other senshi. If the she is protected you could give her color themed senshi (sailor violet, sailor Crimson, sailor ). The color senshi (I know it sounds lame right now but bare with me I have an idea that I'm gonna put on my deviantart account. I'll give you credit for inspiring it of course).

Looking up things about Iris may help. I think she may have had a sector so that could be a cool prism themed weapon too. I think she was also the goddess of messages and some other stuff, so encore rating that could be cool. Sorry if I got carried away and this is to much. This is just really cool and ideas keep coming to me.