Regarding Kaien, although he was the head of the Shiba Clan and Isshin's nephew, Toshiro didn't have much interaction with him.
Isshin rarely mentioned Shiba Clan's matters while in the 10th Division, so Toshiro didn't know much about Kaien, only some things from the anime in his previous life.
However, Kaien didn't appear much in the anime either, only in Rukia's memories. Also, when Rukia and Ichigo went to Hueco Mundo to rescue Orihime, he appeared, but that was the 9th Espada, Aaroniero Arruruerie, transformed, not the real Kaien.
In short, Toshiro's impression of Kaien wasn't deep, and he didn't know much. But he roughly knew the real cause of his death.
The Hollow with special abilities that caused the deaths of Kaien's wife and himself was an experimental creation by Aizen.
In other words, the real cause of Kaien's death was simply an experiment by Aizen.
'Aizen, you truly are a man laden with sins,' Toshiro sighed inwardly.
Before transmigrating, Toshiro quite admired the character Aizen from the anime.
Powerful strength, ruthless personality, grand ambitions… Aizen had countless fans.
But reality isn't an anime, and one can't view this real world through an anime lens.
After experiencing Aizen's threat and seeing people die because of his ambitions, Toshiro couldn't view Aizen's existence as a mere bystander.
As for any former admiration for Aizen, Toshiro had long since thrown it to the winds.
"Rangiku, when did Lieutenant Shiba sacrifice himself?" Toshiro asked Rangiku.
Rangiku thought for a moment. "Hmm, probably a little over a year ago, a few months after you went to Hueco Mundo."
"Is that so?" Toshiro said.
Toshiro then thought to himself, 'That timing... It should be around when Aizen returned to the Soul Society after going to Hueco Mundo.'
Aizen also went to Hueco Mundo earlier, like Toshiro. Although Toshiro didn't know exactly when he returned, it was definitely earlier than Toshiro.
After all, although Aizen could temporarily hide his trip to Hueco Mundo from the Soul Society using Kyoka Suigetsu, if he stayed away too long, the Soul Society might detect anomalies. So Aizen definitely wouldn't stay in Hueco Mundo long.
That Hollow with special abilities was an experimental creation by Aizen. Meaning Aizen subduing Hueco Mundo, returning to the Soul Society, and Kaien's sacrifice likely happened around the same time.
Based on Kaien's death timeline, Aizen probably returned to the Soul Society not long after fighting Toshiro in Hueco Mundo, then created that experimental Hollow, leading to Kaien's death.
'I wonder if Aizen randomly chose Shiba Kaien to test that Hollow's abilities or specifically targeted him,' Toshiro pondered.
Among so many Shinigami in the Soul Society, it happened to be Kaien who died. It was hard for Toshiro not to suspect Aizen did it deliberately.
'Sigh... Whatever. Shiba Kaien is already dead. Overthinking won't change anything,' Toshiro sighed helplessly in his heart.
Currently, Toshiro's strength wasn't a match for Aizen, and he couldn't handle so many matters. He could only try his best to protect those around him, then continue strengthening himself to eventually defeat Aizen.
Afterwards, Toshiro stopped overthinking and sat in Isshin Shiba's captain's chair, pouring himself a cup of tea.
"Rangiku, no matter what, you and Captain are too lazy. How could you pile up this much work?" Toshiro said after taking a sip of tea, frowning at the mountain of documents in the room.
Seeing Toshiro frown, Rangiku immediately defended herself. "Actually, it's not entirely mine and the Captain's fault. After Lieutenant Shiba's death, Captain was too grief-stricken to handle division affairs."
"So? Captain was too sad to handle work. What about you? You're the Lieutenant. Handling division affairs is also your duty," Toshiro said unhappily.
"Um... well... Seeing Captain sad, I was also affected by grief and sorrow, so I couldn't focus on work either. Yeah, that's it," Rangiku nodded repeatedly.
Hearing this, Toshiro felt his eyebrow twitch in anger. This excuse was clearly something Rangiku just came up with to fool him.
But if Toshiro nitpicked with Rangiku, he'd have died from anger long ago. So Toshiro sighed helplessly. "Well, whatever. I won't argue with you. I'll handle half of these documents. The rest are for you and Captain."
After all, leaving all these documents unhandled wasn't an option. Toshiro had to finish them quickly. Dragging it out would affect the 10th Division's normal operations.
Hearing Toshiro would handle half, leaving her and Isshin to handle the other half, Rangiku didn't look relieved. Instead, she looked miserable.
Even though under this arrangement, she'd only need to handle one-quarter, it was still a roomful of documents. Even one-quarter was enough to make her despair.
Originally, she planned to hand all these documents to Toshiro after he returned. Now having to handle them herself, Rangiku felt utterly hopeless.
So Rangiku immediately said, "Um, Toshiro, actually, I just remembered I have something to take care of outside."
Hearing this, Toshiro stared at Rangiku with a 'kind' gaze. "Oh? What do you need to do outside?"
Seeing Toshiro's 'kind' smile, Rangiku broke out in a cold sweat. "Well... um..."
Rangiku stammered, unable to answer. But accidentally seeing Ashido standing aside as a background character, a light bulb went off in her head. "Right! Ashido! Ashido just joined our 10th Division. As Lieutenant, I should show him around and familiarize him with our division. Yeah, that's it."
Being able to improvise this reason, Rangiku felt quite proud of her quick thinking.
"Hmph, no matter what excuse you come up with, it's no use. Today, you must stay and handle the work," Toshiro snorted coldly.
"As for Ashido, you stay and help me with the work too. Let Rangiku see what a model Shinigami looks like," Toshiro said to Ashido.
Toshiro believed Ashido, a model Shinigami, wouldn't be as lazy as Matsumoto Rangiku.
But Toshiro didn't expect Ashido to 'betray' him the next moment.
"Sorry, Toshiro. I think I'll go with Lieutenant Matsumoto to familiarize myself with the 10th Division," Ashido said, glancing at the roomful of documents, then mercilessly refusing Toshiro.