She is actually. Both her and Ichigo are tsunderes. You've seen them both act tough and rough towards each other and their moments where they are nice to each other. If you're asking me Renji is also a tsundere lol.

Tsunderes say the opposite of what they think. Rukia is pretty honest about what she thinks although not aways, she's simply a quiet person so she doesn't shares much with most people.

Isn't it obvious? :)

She's not really a tsundere. Tsunderes are characters who take time to show off their affectionate side, usually acting cold, or rough, to other characters, usually the main character, unless the tsundere is the main character. Rukia doesn't seem to be like that all of the time, she may act that way with Ichigo sometimes, but it's usually when he's being a jerk, or having a whinge, like her often seems to. Narusegawa Naru, from Love Hina, is a much better example of a tsundere. So, my conclusion: Rukia does exhibit tsundere-like behaviorisms, at times, but I wouldn't call her a tsundere.

shes half of it