Shade

Location
Shade is a Djinni that is automatically handed to you as you play through the game, being present on Piers as soon as he joins your party in the town of Kibombo somewhat early on in The Lost Age. Shade is one of three Djinn in The Lost Age - the others being Echo and another Mercury Djinni in Piers' possession, Spring - that is impossible not to obtain.

Description
When Set, it increases base HP by 9, base Defense by 3, and base Luck by 2.

When Shade's battle effect is used, all damage done to all current battling party members is reduced by 60% for the duration of that turn, and this effect is always cast first regardless of the user’s Agility. Shade and the similar Djinn Granite and Flash share the same priority bracket as the Ground and Petra Djinn, meaning that if two units use any of these abilities, the one with the higher agility will act first.

Ability analysis
Shade is extremely useful from the moment it is collected all the way up to the end of the game, because of a very reliable damage-reduction effect. It is typically used in boss encounters, and can give your party a much needed cushion against enemy damage while other Adepts of your party focus on healing dangerously low-health party members. Much later in the game, when Isaac's party joins Felix's along with all their returning Djinn, the Mars Djinni Flash provides a much stronger version of the same effect, but even then Shade is not necessarily considered "obsoleted to uselessness", as both Djinn can be used and alternated between. It could be said that Shade itself obsoletes Granite, though, since that Djinni only protects against 50% damage, making Shade used in Granite's place.

Name Origin
Shade is an area where no light is able to reach. An odd name for a Mercury Djinni, unless it is meant to refer to the coolness in the shade. Or maybe the dark sea-bottom is meant. Alternatively, shade could be viewed as a relief from the sun's harsh rays, similar to how the Djinn offers the party relief from enemy attacks.