SWORD is the Marine Headquarters Top Secret Special Force led by X Drake.
Their operations involve undercover missions to monitor Yonko activity. Koby and X Drake were shown contacting each other to brief on urgent developments.
Other known members of SWORD are:
Prince Grus: A Marine Rear Admiral who was able to easily take down Gotti, a member of the Fire Tank Pirates with a bounty of 90,000,000 Berries.
Hibari: Marine Commander and an associate of Koby.
Helmeppo: Marine Lieutenant Commander and partner of Koby.
The SWORD is not informed of World Government operations and their covert activities. We also know that they use code words to verify secure channels. I’m sure that SWORD is unknown even for the Gorosei. Kizaru was not aware of Drake still being part of the Marine at Sabaody. He is an Admiral and was still not aware of this organization.
I will go even further: The Leader of SWORD is… Monkey D. Garp!
Garp is among the Marines that disagree with the system and how things are run. They aren’t necessarily friendly to pirates, but instead do not believe in Absolute Justice, but Moral Justice.
Koby works directly under Garp and is a member so it’s likely that Garp is aware of the Unit. Also, Garp is old and has a lot of connections with Pirates and Marines alike so he’s in a position to pull strings.
Admirals have to always be on the public eye and the Gorosei always know where and what they are doing. Garp refused to become Admiral so that he could still keep being part of the SWORD.
Garp also used his influence to make Koby climb the marine rank so fast, so he could get his ‘freedom’ and be able to to partake in the secret activists of the SWORD.
In addition to X Drake, Koby and Garp I believe other SWORD members are Sengoku, Fujitora and Smoker. Aokiji has followed in Garp’s footsteps so I think he’s a SWORD member too!
*Theory by seedyProfessor