Genome Codex

The Codex Watch is an item that appears in Resident Evil 7: Biohazard. It is worn on the left arm and first given to Ethan by Zoe after she reattaches his arm that got severed by Mia with a chainsaw.

Purpose
The main purpose of the watch is to display Ethan's health. His condition is represented by an electrocardiogram (ECG) displaying four potential colors (green, yellow, orange, and red) very similar to the early Resident Evil games. In addition to monitoring the well-being of its user, the watch also has a built in phone capable of making and receiving calls. The watch can also track the distance of current objectives with the case in the game being the location of Eveline on-board the tanker. Lastly, the watch is capable of opening the green cases found throughout the game which is necessary to learn about the ingredients for the serum and to create the E-Necrotoxin.

Further Notes

 * It is unknown exactly who manufactured the watch and how old the it is. Given that Mia and Alan were wearing it while transporting Eveline in 2014, the technology is at least 3 years old. And due to the fact Mia and Alan both had one and watch's connection to the green BOW cases, it is reasonable to assume it was developed by the same organization that created Eveline.
 * It is also unknown exactly how Zoe acquired the watch which she later gave to Ethan. It is possible she took Mia's once she arrived at the Baker property or she got one through Lucas's contacts with the organization.