The Lock

The Lock appears as a giant U-Shaped Lock hooked onto the target's heart. As with most Stands, only people with Stand powers can see it. Even the afflicted has no idea it is there. It has no apparent personality. Tamami can also hold The Lock as a tangible item, as shown when he is called by Rohan Kishibe to confirm that Josuke is cheating in their game of dice.

Abilities

The Lock on Koichi's mother and sister

Guilt-tripping: By making someone feel guilt, Tamami can then summon The Lock and hook it to the person. Afterward, their guilt on it is multiplied continuously, represented by The Lock growing bigger and heavier, until the victim becomes so guilty they commit suicide. This does not even require the victim to be guilty about something they did, rather, he is able to make Koichi's mother almost commit suicide by making her feel guilty about her son. The Lock's weight, growing with the amount of guilt, can kill the victim even if they don't commit suicide. However, if the person doesn't feel guilty at all, The Lock doesn't work on them. Alternatively, should the source of a person's guilt be absolved, the Lock will automatically deactivate.

Also, Tamami has says that if someone with a lock on them attacks him, the damage will be reflected on the attacker, but this is never shown.

Lie detection: The Lock is shown to be capable of detecting dishonesty in a person, as shown during Josuke's cee-lo game against Rohan. Though this power isn't elaborated on in much detail, Tamami explains that The Lock will move if it senses honesty, and stay still if it senses dishonesty.