Blood prominently appears in the work of SUDA51, but later becomes a major gameplay element within the games Killer7 and Killer Is Dead. Blood also has a narrative purpose within the former game as it is said to help stabilize the multiple personalities that exist within Garcian Smith, allowing for better control of them, as well as heightening their individual powers.
The blood spilled by a Heaven Smile can be absorbed by Harman Smith's personas and comes in one of two different classifications: "Thin Blood" and "Thick Blood". However, normal attacks cause most of the blood to spill uselessly on the ground, so only critical hits can guarantee a target's blood will be fully collected.
The blood gameplay mechanic recurs in Killer Is Dead, in which it powers protagonist Mondo Zappa's healing, sub-weapons, and instant-kill attacks.
"Thin Blood" is collected in test tubes and is used to power the Smiths' special abilities, as well as heal them while in the field.
"Thick Blood" is collected in a beaker, and is treated like experience points. After accruing enough quantities of Thick Blood, the liquid can be processed by the Mad Doctor through a television in Harman's Room, producing a serum which can upgrade the Smiths' attributes. Upon giving the serum to a member of the Smith Syndicate, their consciousness will heighten, allowing for access to their special abilities and attacks, as well as leveling them up.