HK-51 (Spoiler-Free)

[Sared]

You will need an alt of the opposite faction. It doesn't have to be high level (Level 15 is ok). This is because one of HK 51's components is obtained from the other faction's territory but it is bind on legecy (i.e. you can mail it to your main)

The more the merrier. When hunting down HK-51's components, the more people you have to cover territory, the better. Once a component is unearthed, anyone nearby, regardless of if they are in your party/ops, can loot it. If you scanner detects a component nearby, make sure to call it out so other players can benefit as well!

You will need to run two specific flashpoints as part of the quest.
Only Coruscant and Dromund Kaas compoents are bound to legacy, rest of the components go straight to your mission tab of your inventory and are bound to that character. So yes, you will need a level 50 character to unlock HK-51 but there might be an option in the future to unlock it for other characters in your legacy.

Source, with full walk-through.

[Kokuei]

Spoilers man spoilers. I had no idea HK-51 was coming out or even existed.

[Anamodiel]

Other companion information:

http://dulfy.net/2012/10/12/hk-51-companion-information/

It should be noted that the Weapon companion gifts are HK's favorites. Start stocking up on those now because the price of those is going to balloon up once 1.5 hits. Diplomacy/Underworld Trading/Treasure Hunting should all be able to give you companion gifts!