I don't know exactly how I did this cheat but it worked...
But try this method:

1. Get 'Hazy' Rune from Highland Soldiers. (Rare)
I got it at the very beginning when the Hero and Jowy escape from the "Surprise attack!"

2. Get 'Fire Sealing' Rune in Highland Camp when Jess asks you to check how many provisions they have.

3. Do the Matilda Border cheat where you can recruit Humphrey and Futch early in the game.

4. Make the Hero and Nanami level up. I tried this and they both became from level 9 to level 42. This way, Nanami has 2 rune spaces (Rh and Lh)

5. Attach the Fire Sealing on Rh and Hazy on Lh.

6. Make sure Nanami is always in your party.

7. Get the 'Kindness' Rune on the dock at Lakewest Town.

8. Attach the 'Kindness' Rune to Nanami's weapon.

9. Refine your weapons until level 11.

10. Save your game at the inn at you castle.(Note: Don't save at the journeyman crystal or this whole process is ruined.)

11. Restart the game and you'll see Nanami with 999 attack power.
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NOTE:
This only works on Nanami.
Not the Hero.
The good thing about this cheat is when you unite with the hero (Family Attack),
It is really strong.

Another thing, this cheat is completely worthless after you overtake Matilda because Nanami would die there.

At the name entry screen go to Determine and press L1; then a name will be randomly selected.



If you don't like that name press triangle until it is totally deleted, then press L1 again until you find a name you do like...or just put in your own name.