The altar resembles a shallow basin on a trio of long legs. In the space between each, the basin is engraved with stylized images that presumably describe certain events being memorialized. To the left, a figure carrying a broadsword-like weapon faces off against a swirling storm. To the right, the same figure floats in an empty space. The third space (on the opposite side) reads:

Riku
Keyblade Master

The effects of leaving an offering are listed here.

About halfway through the week, once Riku has learned of the effects of offerings and discovered his own monument, the delphic tripod will be covered by a dingy drop cloth as his way of preventing others from being... well, inconvenienced.

Riku prefers to keep himself busy and can be seen doing menial tasks that mainly deal with solving problems around Beacon, such as patching up potholes with flat stones and wet sand, some light carpentry to fix doors and leaking rooftops, moving furniture and scavenging for materials around the outskirts of the currently known territory.

His routine shifts a little given the mysterious arrival of the altars and memorials. If Riku is troubled by his own experiences with them, he isn't inclined to share his thoughts. This does nothing to prevent him from checking in on those who seem distressed.

Riku may not have much - like all of them, he arrived with his own meager belongings - but sometimes all a person needs is someone else to be present. Someone who takes a moment to ask:

"You okay?"