To unlock character viewer and trailers in the extra feature, you must locate and destroy the Yellow Capsules in the individual Extra Missions using a gunshot to destroy it. Each one will unlock something different.

Bosses
Yellow Capsule in Mission 28: Cyclopean Causeway

DC Presentation Trailer
Yellow Capsule in Mission 10: Misslebreaker Melee, I-7

Deepground
Yellow Capsule in Mission 30: 100,000 Needles, L-7

Jump Festa Trailer
Yellow Capsule in Mission 13: Rains of Gehenna, J-4

Monster
Yellow Capsule in Mission 11: Collector's Mind, D-5

Omega
Yellow Capsule in Mission 44: Deepground, J-8

Others (Other Character)
Yellow Capsule in Mission 10: Misslebreaker Melee, G-10

Retail Display Trailer
Yellow Capsule in Mission 22: Shinra Manor Prime, I-7

Shinra
Yellow Capsule in Mission 20: Deep Tower, K-8

The WRO
Yellow Capsule in Mission 1: Zephyr Heathlands, G-11

Tokyo Game Show Trailer
Yellow Capsule in Mission 3: Wastelings, B-9

Tsviets
Yellow Capsule in Mission 41: Drumline, J-2

Vincent
Yellow Capsule in Mission 36: Black Widow Tetra, I-5