Perks
Perks are passive abilities that customize your soldier in multiplayer. Tier 1 includes perks like Bandolier (more ammo) and C4 x2. Tier 2 offers Stopping Power (increased bullet damage) and Sleight of Hand (faster reloads). Tier 3 has Extreme Conditioning (longer sprint) and Dead Silence (silent movement). Choose perks that synergize with your weapon and playstyle.

Killstreaks
Killstreaks are rewards for achieving consecutive kills without dying. The 3-killstreak UAV reveals enemy positions on the mini-map. The 5-killstreak Airstrike allows you to call in a bombing run. The 7-killstreak Attack Helicopter provides aerial support. Using killstreaks effectively can turn the tide of a match.
While Call of Duty 4 focuses on grounded military perks, Code Vein utilizes a similar passive buff system through Blood Codes to significantly alter character attributes.