I need some help with "tactical" or defense knives?

I do not NEED this knife per say, but I would very much like something that I can learn to defend myself with as an added precaution late at night. Also, I think knives are very cool… But anyways, the knife should not be visible, as I will be going places with it, and am not allowed to walk around with a 7 inch blade out. Hahaha, so yeah. The rough guidelines are as follows:

1: Needs to be able to be used in combat fluently or easily, and/or be so fiercely awesome that it frightens the opponent away.

2: Needs to be concealable, up the sleeve, in a glove, something!

3: I would prefer it look really unique and different, not just some knife.

4: Around 50$.... negotiable.

Please tell me if i missed anything and as a last thought, I’m just adding this in, does anyone know anything about trench spikes? They are a subject of great interest to me, and any/all information on good ones and bad ones and recommendations on them would be greatly appreciated. (Also, this would be the ultimate weapon for me, but am I the only one that thinks that a trench spike, with a retractable blade, would be the sickest thing ever? Like brass knuckles with a spike that pops out!!)

Anyways thanks for any/all feedback!