Sony OWNS the Ratchet and Clank IP. They have Insomniac handle it, but could pass it to another studio if they saw fit.
Helghast are the antagonist race of the Killzone IP, and have had a presence in the PS brand for a long time.
Sackboy has had a stong presence on all Sony platforms from the PS3 onward, though he has lost steam with LBP3. He has been used often in promoting the PS4 overall though.
Just because you may not be privy to an aspect of Sony IPs does not mean that those options are null. Most people know about them, regardless of your personal perception.
If you read my post, I never said Sony does not OWN Ratchet and Clank, I said Sony don't MAKE Ratchet and Clank. There's a difference there. Insomniac made Ratchet and Clank what it is not Sony, Sony just own the IP, a bit like if MS made Alan Wake there mascot even though Remedy made it, it doesn't work. If anything, Ratchet and Clank works better as Insomniac's mascot.
To me, a Mascot needs to be internal, it needs to be created from the inside and represents the brand as a whole, not relying on 2nd or 3rd party devs to make your mascot for you.
Lets be honest here, Helghast? I wouldn't be the only one who doesn't know who that character is.. you cant just throw in a character because they were in a franchise, the character needs to be recognized, they need to be popular outside of gaming. Mario, Sonic, Master Chief are all well known characters inside and outside gaming that not only represent there brands but they lead by good examples. This is why Sackboy and Helghast don't cut it, I would say Knack has a better chance at being the mascot. Even characters like Kratos being a violent character doesn't really make for a great mascot either. Mascots are versatile and flexible characters. Having a character that rips hearts out of his enemies wouldn't be the best idea, however out of that list you mentioned, I would say Kratos comes the closest. Reason I wouldn't put Drake as the mascot is because his not a long term prospect, Kratos is.