Mythril Bag
Mythril Bags (ミスリルのふくろ, Misuriru no fukuro?, lit. Mythril Bag) are key items that are placed in the inventory by the first two games in the Golden Sun series to represent that the Elemental Stars are being carried in the party's inventory. They only serve story purposes and cannot be used to affect the gameplay. The only Mythril Bags found in either of the Golden Sun games were originally in Kraden's possession, and it is not clarified how he got them. On the other hand, it is implied that he takes them out of his cottage and brings them along as he and Isaac secretly enter Sol Sanctum at the start of the first game because he hopes to find the supposedly real Elemental Stars within the temple and take them with him to study on behalf of his master, Babi.
Note that in the first game, after Kraden gives Isaac the four Mythril Bags and instructs him to fetch the four Stars in the Elemental Star Chamber, they are replaced in the inventory one-by-one with each Elemental Star key item as Isaac carries out his task. Following this sequence, the Mars Star will exist in the party's inventory throughout the rest of the game as a key item that similarly cannot be removed, and it will appear as itself rather than as a Mythril Bag described in-game as containing said Star.
In contrast to this, Mythril Bags described as containing specific Elemental Stars will exist in place of the actual Elemental Star key items themselves throughout The Lost Age. Felix will start the game holding a Mythril Bag described as holding the Jupiter Star — and rather uniquely for any item, Felix will be incapable of Giving this item to any other party member's personal inventory, regardless of which version of the Mythril Bag he retains at any given point.
The Jupiter Star-holding version of the Mythril Bag will remain in Felix's inventory up until especially significant cutscenes take place near the end of the Jupiter Lighthouse dungeon, where it will temporarily be replaced with a different Mythril Bag item that is described in-game as holding both the Jupiter and Mars Stars. That bag will soon be automatically replaced again during another cutscene, this time by a version of the bag that is not described as holding anything at all.
Near the end of the game, following a boss battle in Mars Lighthouse, the party will be allowed to take the Mars Star off the ground. At this point, Felix's empty Mythril Bag will be replaced with a version described as holding the Mars Star, and Felix will be required to Use this Mythril Bag from his inventory on a dragon head statue when prompted to "heat up" the Lighthouse's mechanisms.
It can be speculated that the reason The Lost Age uses Mythril Bags to represent Elemental Stars being carried in the inventory for story purposes instead of having the Elemental Stars themselves directly populate the inventory like in the first game is because of the previously alluded scene where Felix must carry two separate Stars in his inventory for a short time. Had the game not used one Mythril Bag item to represent both Stars, the game would have had to place the Mars Star into an extra inventory spot, which would have caused problems if all fifteen of Felix's inventory slots were already filled up at that time.
Felix's bag being impossible to remove from his inventory also prevents the player from placing the bag in any party member's inventory prior to any occasion in the story where that character leaves the party (namely, when Felix potentially leaves Piers behind in Kibombo after meeting him for the first time there, when Piers leaves during the party's exploration of Lemuria, and when Jenna and Sheba are left behind following the scene at Jupiter Lighthouse when Felix gains the bag containing two Stars at once).
At any rate, the Mercury Star and Venus Star are not replaced by Mythril Bags if they have been hacked into the inventory because they are not meant to be items transferred or used in any way in the second game.
