Tsukiumi is Sekirei #09.
For a good portion of the first season, Tsukiumi remained unwinged. This is due to two reasons:
One: Tsukiumi desired to be the strongest sekirei without bonding to an ashikabi.
Two: And this is the silly one. Tsukiumi was under the false impression that to wing a sekirei the ashikabi had to have intercourse with the sekirei in question. Just one of those things that seems to come standard in most ecchi animes.
When she and Minato share a dream and their connection is revealed, Tsukiumi states quite clearly that she desires nothing more to kill him. However, she is physically unable to do the act as she is overcome with "feeling" around Minato. In this same encounter, Seo threatens to wing her himself, and to make her his personal slave. At this point Minato, already having bodily tackled Tsukiumi out of the way of a massive lightnig attack that would have deactivated her, steps in and stops Seo. Tsukiumi then submits to the feeling inside of her and is winged by Minato. She then freaks out as she discovers that she is now one of four sekirei.
Tsukiumi controls water with a great deal of flexibility in combat, using a variety of moves that she always announces before performing. As of now she does the following:
Water celebration: standoff move that just drenches whatever she is aiming at. Miya claims that it leaves her open and is quick to prove it.
Water sword: exactly what it sounds like. Forms up around her right arm rather than being held like a real sword.
Water arrows: small, dagger shaped bits of water, highly inaccurate, but good for area of effect attack.
Water Dragon: currently her trump card. Litterally makes a dragon's head out of water which chases her opponent until it hits.
In terms of personality, two words can describe Tsukiumi's "normal" demeanor. Stubborn, and aggresive. And she will not accept defeat in anything, including something so innocent as a shopping race. As Minato's sekirei use such contests to decide who gets to spend time with Minato, and Tsumiumi always loses at them, she rarely gets to spend much quality time with Minato. This leads into discusion of her other side. Shortly after she becomes Minato's sekirei, and with increasing regularity afterwards. Tsukiumi starts to show her softer side the more neglected she feels. During the first time this happens where Minato actually sees her doing this he actually realizes for the first time just how beautiful she really is. Tsukiumi however does not like it when she lets herself break down even slightly and is seemingly infuriated by his stare. Later, after being soundly spanked (figuratively speaking) by the landlady, Miya, in a training session, Minato notices just how scratched up she is, cue another emotional moment when we see just how deep the feeling of neglect runs, Tsukiumi is very nearly crying as she claims that Minato hasn't even looked at her since she came to live with the rest of them. This is followed by what might be the most openly erotic moment in the anime so far, she physically moves Minato's hand from her shoulder to her left breast, speaking to him in the same tone as before "don't you... want to hold me?" Shortlly afterwards she catches herself again and storms off. It is moments like this that make the watcher start to feel quite sorry for Tsukiumi. Still further on, during one of the meals, and in keeping with the nature of the conversation, Tsukiumi and Minato get caught staring at each other. The moment is absolutely perfect and they start to lean toward each other in what would have been the first non-winging kiss that any of Minato's sekirei had done with him. The romantic moment is however, rudely interupted by Musubi projecting a loud, and extremely disruptive bear image in between the two before practically jumping on Minato. It rather makes me wonder about Musubi having severe jeaousy issues.