Complete the game and then search at the indicated locations to find the corresponding characters to have a final interaction with them.

Rima:
On top of her towerin the jungle.

Infantry/Army:
At the Bladehenge beach party.

Kill Master:
On the highway north of Bladehenge patrolling on his motorcycle.

Thunderhogs:
At the Kill Master's Lair. You can drink with his bassists and comment on Lemmy's bass playing ability.

Fire Baron:
In the jungle directly north of the tree island at the bottom of the last continent. He can be found to the left of the main road as soon as it turns, traveling south to east.

Lita:
Go to the graveyard located in the first grassland portion of the map near Mount Rockmore. You will find Lita kneeling by her fallen relative's headstone. Press 'Interact' to view a brief cinematic where the time of day will shift until you press the 'Interact' button again.

Ophelia:
Return to the big stone cross in the grasslands area which is located to the left of the large sword on the map. Ophelia is sitting in front of the stone cross (near the landmark viewer). Press 'Interact' and Eddie will ask if she wants to make out and a cinematic will play of the two kissing until you press 'Interact' again.

Mangus:
On top of the large sword in Blade Henge (use the nearby Landmark Viewer).

When you complete the following mission chains the corresponding extra upgrade will become available.

Unlock Spike-Quilled Axe:
Complete ALL the Hunting secondary missions to get this lethal weapon.

Unlock Triple Nitro:
Complete ALL the Racing secondary missions to get increased nitrous.