Striker has been explained on the board a million times, if you don't know what it is it's because you're not listening...
In the 4-2-5 Defense the Striker is a HYBRID LB/S, that means it is a multiple-use position, it can be another LB on run downs or can be a drop back Safety in coverage on passing downs.
Striker is not something that only we use or have, literally every 4-2-5 has a striker & other Defenses have a similar position for their Nickel sub-packages; UF has the same position as well even though they run a 3-4 base Defense, they have the Star position which is just a Nickel Safety that plays down in the box vs Run, but can also drop back in coverage & cover TE's, WR's RB's out of the backfield etc, in Zone & Man looks. Chauncey Gardener-Johnson was their "Star" position.
Bama has the Star position as well, Shyheim Carter played Star for them, he was a CB they converted to Safety. Same with Tennessee & UGA they also have a Star position.
Oh St has a similar position as well being that they play 3 CB's & 1 Safety instead of the traditional 2 Safeties in their 4-3 scheme. Oregon does too.
San Diego St had one of the best Defenses in the nation last year & they run the 3-3-5 & have the "War" position.
Oklahoma & TCU run the 4-2-5 like we do, Oklahoma uses more 3 down lineman sets than us but it's basically the same defense. Muschamp at South Carolina runs the 4-3 but uses their Nickel like a Striker in their scheme.
Lots of teams use it & have a player on their defense that has versatility in their assignments which allows them to stay on the field vs no huddle offenses & can cover multiple spots.