The First Seven
Five of the first seven Djinni are missable. Once you progress past a certain point in the game, a whole section of the world becomes inaccessible. Be sure to get each of the Djinn before you leave!
Flint, Eart h - This spirit was Isaac's very first Djinni. Flint talks a great deal, but is both knowledgeable and nice, so no one complains. Ability: Attack with a blow that can cleave stone. Flint is unmissable; he joins you as your first Djinni in the tutorial as you leave Goma Plateau.
Forge, Fire - This bolstering spirit is encountered at Patcher's Place. Forge's blazing flame stirs allies to greater ferocity in battle. Ability: Boost party attack with flame's fury. Forge is obvious sitting on a cliff above the Psynergy Training Grounds. You'll have to push several blocks around to allows you to jump from the treetop to the cliff to nab him. Forge is Permanently Missable
Gust, Wind -This fleet spirit is first encountered at the Goma Highlands Road. Gust's tremendous speed always gets the jump on an enemy. Ability: Buffet foe (perhaps twice) with gale force. You'll probably see this Djinn on the road to Carver's Camp and won't be sure how to get him. You'll need to actually enter Carver's camp and circle around a house, casting Move on a pillar to allow you past. Explore the area using the gondolas and you'll find it. Gust is Permanently Missable
Chill, Water - This spirit of intense cold is found at Carver's Camp. Chill's attack renders enemies brittle and vulnerable. Ability: Shatter a foe's defense with numbing cold. This one is easy to miss. There's a tree you can climb down in the lower right corner of the camp, and a vine you can grow to climb even further downward, to a small area that has nothing but the Djinn. Chill is Permanently Missable.
Fever, Fire - This spi rit of dreams and hallucinations is found near Konpa Gate. Fever's power overheats a foe's brain, causing delusional thinking. Ability: Damage and delude a foe with a heat haze. Fever is unmissable as far as I know. When you're heading towards Konpa from Carver's Camp, you'll automatically get in a battle with an unnamed Mars Djinn. Defeat it and Fever is yours. If, however, you somehow manage to miss the encounter, Fever would become permanently missable.
Flower, Earth - This flower spirit is found by the gate to the Konpa Ruins. Flower's sweet scent has a refreshing and healing effect. Ability: Refresh allies and restore HP. You should see Flower among some flowers when you have the man open Konpa Gate for you. Once past the gate, when you go to find it, Flower will hide among some flowers. Use Whirlwind to lift them and reveal the poor Djinn. Flower is Permanently Missable.
Jolt, Wind - This lightning spirit is first encountered at the Konpa ruins. Jolt generates a shock that can bring an unconscious ally around again. Ability: May revive a downed ally with sharp shocks. Atop one of the rooftops at the peak of Konpa Ruins, you'll see Jolt. Using the Grip psynergy on the poles, you can pull yourself to the rooftop, but Jolt will run when you get close. You'll have to use the Grip psynergy when it's between you and a pole to pull yourself into it, starting a fight. Jolt is Permanently Missable.
Any Djinni missed once Rief joins your party (after Blados destroys the exit to the cave) are gone forever. You will never be able to return to the starting areas.