Flint

Location
It is automatically acquired once you leave Vale and enter the overworld for the first time, where a cutscene has Flint talk to you and give a tutorial on how to use Djinn. It is nearly identical to Venus Djinni Echo in its speaking role as the "tutorial Djinni" of its respective game. It will join you no matter how many times you attempt to refuse it joining you, but its various responses may be amusing.

Description
When Set, it increases the Adept's base HP by 8, base PP by 4, and base Attack by 3.

When Flint's battle effect is used, the user strongly attacks a target with a Venus-based attack equal in power to the user's normal physical attack increased by 60%. This effect is identical to the battle ability of Echo in The Lost Age.

Ability analysis
In the first game, Golden Sun, Flint is the one Venus Djinni that deals an attack enhanced by a damage multiplier rather than a flat addition to resultant damage. This makes it consistently strong throughout the game, as it scales up with your attack power. The other two attack Djinn of the Venus element, Sap and Bane, give straight damage, and while they may end up being more valuable to an endgame party that is not of a particularly high level (because they have secondary effects alongside the damage), a highly-leveled up party will inevitably find this to deal sharply more damage. Aside from Gust's chance to deal double damage whenever it is used, Flint has the highest damage multiplier of the attack Djinn in Golden Sun, and is especially ideal when used by a high-level Isaac equipped with all seven Venus Djinn and the Gaia Blade and Warrior's Helm.

In the second game, The Lost Age, as soon as Isaac's party joins Felix's party along with their returning Djinn, you have easy access to the Venus Djinni Geode at Atteka Inlet, which obsoletes both Flint and its identical counterpart Echo with a Venus-based attack equal in power to the user's normal physical attack increased by 90%. While none of the various other Djinn with damage multipliers are even as high as Flint and Echo's 60%, Geode's 90% makes usage of that Djinni very common and usage of even these lesser Djinn quite rare.

Name Origin
Flint is a type of stone normally used to make arrowheads and other weapons. Flint is also a way to make fire by striking it against Steel.

Solo probably refers to the fact that he strikes once; this is opposed to the first Djinni in the second game.