Nankotsu is one of the oldest ninja clans in Tanmen, providing spies and assassins for at least 400 years. The clan derives its name from the specific way of training the members receive and things they're capable of doing.
Kids are usually purchased from poor farmers at a very young age and then stretched daily to increase their pain resistance and flexibility to the point where all of their bones seem to be made of cartilage, which in fact gave the name to Nankotsu (lit. soft bone/cartilage) Clan.
The special training techniques make their bodies semi-immune to pain, making them effective spies because even if they get caught (which actually almost never happens), it would be physically impossible to make them talk as their bodies would sooner die from the damage taken than they would reveal any information as they have this special ability to ignore the pain signals from the nerves. This ability is polished daily during the training and demonstrated every evening to inspire younger trainees and entertain invited guests and clients.
They can use the flexibility gained from the training to squeeze through the smallest holes in buildings and hide themselves in the least noticable places while spying on their targets. This ability is especially effective while they're still young and tiny, so the kids are trained to be limber and quick but to avoid enemy soldiers at all costs because they simply wouldn't be strong enough to fight yet. If they manage to survive to the age of 14-15 as simple spies, they're promoted to assassins, and trained to use a variety of weapons. Assassins also grow long hair and often bleach it, though that's something for another picture ^^
Like I was saying, well-trained spies would often have to demonstrate their skills to younger kids, and usually several high Yakuza bosses are invited to see the show as well, so that the potential clients can see the advantages of the Nankotsu techniques. The cruel Yakuza chiefs are usually very pleased with the things they get to see during such demonstrations and would often sign the contracts immediately.
So this is young spy Kagemaru a.k.a. Kage-chan, getting his back flexibility polished by his Master ^^;