When you killed Jas and watched credit, it didn't give you an option to save and reload for the New Game+, it is because you cannot start New Game+ when the story is completed. New Game+ in this game is a time machine that send you back in time to replay the game while retaining all of your assets.

1. Reload your save game before Jas.
2. Teleport to "Shangri-La -> Altar".
3. Offer the "Tachyon Medal" you obtained from the "Sanctum - Laboratory".

Note: I highly recommend you save in another slot for the NG+ so that you do not lose your progress in the first playthrough.

1. Second Growth Map: Unlock after you completed your original Growth Map (not necessarily in NG+).
2. Higher Level Enemies: All enemies have your level + 30.
3. Chests containing extra item SP Capsule: Some chests give extra item (always SP Capsule) when open.
4. Music Box: There is a new chest containing Music Box in the room just outside the Mini Toy Box.
5. Growth-Items Trade Machine: There is a new machine just outside of Bernard's Laboratory (Hidden Valley), it trades in your growth items (items that unlock path to in Growth Map).

• Your first priority when starting NG+ is to dump SP Capsule on Leo just enough that he can complete his Growth Map. Once completed, a second Growth Map will be unlocked.
• You'll see 5 passive skills in the middle of each section. I highly recommend you dump more SP Capsule to unlock Lock Picking, Encounter Down, and Field Speed Up.
  • Lock Picking allows you to open locked chests without needing a key.
  • Encounter Down reduces the chance of encountering enemies which help speed up your game.
  • Field Speed Up allows you run faster. 
    Note 1: These passive skills activate only when walk as Leo. If you want to walk as Kina have these skills activate, you'll have to unlock second Growth Map for Kina. 
    Note 2: Everyone has the same Second Growth Map.

Some chests have extra item SP Capsule when open. So during your NG+ run, you'll rack a lot of SP Capsule as well just enough to unlock second Growth Map for everyone at the end.

• All enemies have their original level + 30 which mean they are, in theory, stronger but they are actually not until late game when they start hitting harder. You shouldn't have any problem killing them.
• All bosses are almost instantly kill by 2 or 3 turns when using Leo's combo "Concentrate" + "Desperation" + "Samidare (Fire+2)".
• During my run on NG+, I didn't even bother killing monsters so I turned off the Dimengeon, and use "Escape" instead because it's faster. At the end of the game, I was around level 90 of all characters. The key is you take turn to equip everyone with Unfamiliar Stone and Vibran Secret Stone when killing bosses.

There is a new chest in the room just outside of the Mini Toy Box, it contains Uematsu's Tape Deck. This item unlocks a new menu in the start menu screen, it's a music player with a library of all music in the game. It is also one of the achievements to unlock.

There is a secret dungeon inside a well in En - Old District. It's a labyrinth just like one of the trial of elements but WITHOUT a map. I didn't know at first and I was lost, had to restart the game because I couldn't find a way out nor teleport because I didn't have the warp device yet. So I suggest you run this dungeon after you got your warp device.

I spent a few hours drawing the map so you don't have to suffer I like do. You should always count your steps when you move, then track left turn, right turn or U turn according to the map shown below:

Items inside the secret dungeon:

• Order Motes
• Chaos Clay
• SP Capsule
• Elixir
• Pearl XXL

This is a new machine outside of Bernard's Laboratory. You can trade your Growth items (items that unlock path to skills in your Growth Map) for  5 SP Capsules.