This list is what you can upgrade to make Sebastian himself better. Note that upgrading everything here nets you the "Hard Body" Trophy and/or achievement.

On Casual and Survivor, the Life Gauge and Syringe Recovery are helpful buys, but note that buying the life gauge will fill up whatever life you are missing at the time, so treat it like a free health refill. On Nightmare and above, the maximum sprint time is ESSENTIAL

Melee damage is, well... never any good.

LIFE GAUGE

• Level 2 (110 Life): 2,000 Green Gel
• Level 3 (120 Life): 5,000 Green Gel
• Level 4 (130 Life): 10,000 Green Gel
• Level 5 (150 Life): 25,000 Green Gel

MAXIMUM SPRINT TIME

• Level 2 (4 seconds): 3,000 Green Gel
• Level 3 (6 seconds): 6,000 Green Gel
• Level 4 (8 seconds): 9,000 Green Gel
• *Level 5 (10 seconds): 30,000 Green Gel
  * = Level 5 also adds in an increased stamina regeneration effect.

MELEE DAMAGE

• Level 2 (120%): 2,000 Green Gel
• Level 3 (150%): 3,000 Green Gel
• Level 4 (180%): 6,000 Green Gel
• Level 5 (240%): 18,000 Green Gel

SYRINGE RECOVERY

• Level 2 (36 Life Restored): 5,000 Green Gel
• Level 3 (42 Life Restored): 9,000 Green Gel
• Level 4 (48 Life Restored): 15,000 Green Gel
• Level 5 (60 Life Restored): 30,000 Green Gel