↑Depends on the augments in the character's battle menu afterward.
When Rydia rejoins the party the second time, she will be at level 30 if Child Rydia's level was lower or equal to that level. If Child Rydia's level is higher than 30, Rydia will retain it. Finally, if Child Rydia's level is over 70 and has a specific level up (thanks to the Augment ability system), she will retain that level however she will also lose all stats bonuses. The stats bonuses applied are those from her default battle menu as an adult, meaning she will get bonuses for having Attack, Black Magic, Summon and item.
↑Depends augments in character's battle menu afterward
To reach the max 99 points in her stats, Rydia needs 63 points in Strength, 66 in Speed, 65 in Stamina, 27 in Intellect and 45 in Spirit.
Rydia can use Black Magic and learns every spell, up to and including Meteor, but she learns them later than Palom due to her emphasis on Summon Magic. As a child, she can cast some basic White Magic. When she returns to the party as an adult, she has forgotten these spells.
In the 3D remake versions, Rydia can learn more White Magic as a child, and learns her later Black Magic at different levels. Rydia joins the party at Level 7, while she joins at level 1 in all other releases, and thus knows several spells already. When she is reverted to a child during her Lunar Trial in the Advance and Complete Collection releases, she can cast her White Magic while retaining her Black Magic.
When Rydia rejoins as an adult in the Dwarven Castle after beating Golbez (2D and 3D), she has Fira, Blizzara, Thundara, along with Cura. At this point, she no longer has the White Magic command and cannot use her White Magic spells and thus Cura can never be used outside of her trial in GBA and PSP versions, when she'll lose Summon Magic and temporarily regain White Magic.
In all releases, Rydia learns her Eidolons the same way.
In the Game Boy Advance and PlayStation Portable versions, there is a loophole the player can use to equip the adult Rydia with a staff: when she transforms into a child in her Lunar Trial she regains her White Magic and the ability to equip staves, and will not remove any equipped staff she is given when the trial ends and she turns back into an adult. However, once the staff is removed it cannot be re-equipped without repeating the trial. This loophole also works with the White Robe.
Rydia enters a cave filled with mist. She turns back into a child, and Ifrit, Titan, Ramuh, and Shiva abandon her. The party must find them and defeat them to regain them. After defeating them the mist dissipates, and the party fights Lunar Dragon. After defeating it, the player gets the Mist Ring, which improves Rydia's Mist Dragon summon and grants the party the Blink status.
The Lunar Trials are available in the GBA and PSP versions.
Rydia calls Sylphs to drain enemies of strength and power up Yang. Casts Haste, Protect and Shell on Yang, and heals HP equal to total damage done to enemies. Can be used even if the Sylphs summon has not been recovered.
The Eidolon Chocobo can be missed if the player does not pick it up inside the Chocobo Forest during the Gathering's Tale: The Moon's Gravity. It is located on the island in the middle of the ocean between Troia and Kaipo.
Many Eidolons can be permanently missed if killed during their battles.
If Rydia has recovered Leviathan and Asura as summons, Bahamut will turn on the Mysterious Girl and destroy her before returning to Rydia, but if not, Bahamut must be slain and will not be available to be summoned.
Several items are only obtainable in Challenge Dungeons; once the final section of the game is started, anything that isn't imported from those is permanently lost.
Rydia's Challenge Dungeon is a three floor cave. Many doors are actually Trap Doors, while several walls are Demon Walls and cause doorways or staircases to appear when defeated. The first two floors are mazes of doors, while the third floor consists of several platforms that are connected by ropes. On each platform only one of the ropes can be climbed safely, the other two trigger a scripted encounter. The dungeon is accessed by speaking to Challengingway in Agart. During some plays of the dungeon, certain doors will be locked. Though the player will always be able to reach the end, some side passages with treasure may be blocked off.