Bethune's trial in Tokyo District Court is scheduled to finish on Monday after three days. A verdict is expected in late June.
The member of the Sea Shepherd Conservation Society is being tried on five criminal charges, three related to boarding the Japanese whaling fleet's security vessel in the Southern Ocean on February 15.
He has conceded four of the charges but has contested an assault charge over allegedly hurling a bottle containing butyric acid, or rancid butter, on to the Shonan Maru 2 and injuring a crew member on February 11, as the protesters tried to disrupt the whaling activities.
He boarded the Shonan Maru 2 and presented its captain with a letter seeking $3 million in replacement costs for the trimaran Ady Gil. He captained the trimaran before it sank after a collision with the Shonan Maru cut off its bow the month before.
In court today, Japanese whaler Takashi Kominami, 24, said he suffered facial chemical burns from the butyric acid bottle thrown by Bethune.
"I'm angry," Mr Kominami said.
"I think he doesn't feel remorse at all. I call for severe punishment so that he feels remorse."
He said that he had seen Bethune holding a launcher and thought the New Zealander fired the projectile that hurt him, although he did not actually see him launch the bottle.
But one of Bethune's lawyers, Dan Harris, said the evidence was clear that the bottle was not thrown at anyone in particular.
"It was thrown at the ship to try and get these people off the deck because it smells so badly, and it hit a part of the ship, and if it did hit anyone, it's only because of bad luck ... some may have blown on people," Mr Harris told Radio New Zealand.
"My understanding is it's [the trial] gone pretty well, and that's why I'm pretty confident that this charge is not going to be taken very seriously by the court in terms of believing that anyone was badly injured, because nobody was."
During his testimony on Monday, Bethune will argue that what he did was not illegal, and raise the issue of whaling.
Mr Harris did not expect repercussions over the legal action Australia will take next week to stop Japan whaling in the southern ocean.
"Our hope, and our belief is that this is a legal proceeding, not a political proceeding. Japanese judges tend to be very well educated in law, they tend to be very legalistic, and that is how we're hoping they rule," he said.
However, it could attract more extreme nationalist protests outside the court on Monday.
Speaking to the Kyodo newsagency while in his Tokyo detention prison, Bethune asked the Japanese not to think of him as a terrorist, and has said he may not continue his anti-whaling work.
"I have nothing against Japanese people. Don't think of me as a terrorist."
Asked whether he would stay with Sea Shepherd, which he joined last July, he said: "I don't know. It's almost a year now with no income."
All of the crew aboard the US-based group's vessels worked as volunteers and did not get paid, he said.
Mr Bethune, an engineer and conservationist, said the "eco-terrorist" label "sits very uncomfortably with me".
He mortgaged his house, sold all his shares in a high-tech startup firm he founded, and took on heavy debt to build the Earthrace, which was repainted and renamed the Ady Gil, for a record circumnavigation of the world as a bio-fuel speedboat in June 2008.
