The Hidden Blade is the Assassins' signature weapon, a retractable blade under the wrist that Jacob and Evie each wear in their Assassin Gauntlets from the first mission. In Syndicate it is your tool for quiet kills, air assassinations and double assassinations rather than open fighting.
Stealth skills such as Air Assassination and Double Assassination run through the blade. Assassin Steps, Stealth I to III and Cover Assassinate-style perks all make those kills quieter.
The same gauntlet design stays in use for decades. Jacob's apprentice Jack and his granddaughter Lydia both wear it, and by the late 1800s the Hidden Blade is still standard Assassin gear.
Brass Knuckles
Brass knuckles are Syndicate's brawling weapon, a metal grip over the fist that turns every punch into a heavy blow. Jacob starts the game wearing a pair, and they suit his straight-at-them fighting style, though Evie can equip them too.
Cane-Sword
The cane-sword is a walking stick with a concealed blade, and in Syndicate it is the melee weapon to pick when you want to stun and control a fight rather than end it fast. Evie starts the game with one, and the twins' version hides a second, scythe-like blade in the sheath.
Kara
Karas are steel or iron bracelets that Evie wears as her main close-combat weapon in the Jack the Ripper expansion. The Indian Brotherhood used them as arm guards and as knuckle-dusters, so they hit much like brass knuckles.
Kukri
The kukri is the deadliest of Syndicate's three melee weapon types, a curved Nepalese chopping blade that Jacob and Evie can both equip. Pick it when you want enemies to die quickly; it trades away some of the stunning power a cane-sword gives you.