Sumeru Act 5:
This voice line is the first time Alhaitham mentions not living alone. Dehya sounds surprised at Alhaitham rather random comment, prompting fans theorize, that this is an instance of Alhaitham sulking or thinking out loud. Especially with the way he deflects right after that.
Afterwards, after the main events of the Archon quest finished up, does Nilou ask the traveler to deliver a letter to her friends to make them join the celebration gathering.
An NPC named Geoff, who is apparently very fond of Gossip, tells the traveler about Alhaithams whereabouts, and that Kaveh already went looking for him before the traveler. Letting us also know that Alhaitham and Kaveh are quite infamous for their terrible relationship.
Soon after, we see Kaveh and Alhaitham bickering in the House of Daena. It has been noted, that in this exchange, Alhaitham and Kaveh seemed quite a lot more frustrated with each other than in later quests and Events. Alhaithams tone of voice also changed significantly when speaking to Kaveh than when talking to any other character. A detail, that has been confirmed by both the English and Japanese voice actors to be a deliberate choice.
During the exchange, Alhaitham compliments Kaveh. It sounds sarcastic, but after learning more about Alhaitham in his character stories, this might have been a genuine compliment by Alhaitham that Kaveh misunderstood.
A fantastic analysis of the House of Daena scene and why Alhaitham was so snappy has been made by starcurtain on tumblr. The user makes the hypothesis that Alhaitham being so harsh on Kaveh might have to do with Alhaitham being genuinely worried about Kaveh’s whereabouts and wellbeing. After all, Kaveh was sent on an expedition to the desert by the desert by the Kesharewar sage that had been working with Azar. The worst thing about this, is that this could have been a way to make Kaveh disappear, like the Academia did with other scholars in the past.
Another funny thing that has led to a lot of fanart and discussion, is this small scene of Kaveh presumably checking Alhaitham out.
To-Do: The exchange between Alhaitham and traveller, The secret exchange between Alhaitham and Kaveh
Nod-Krai Act 4:
The Traveller talks to Dori after we face Rerir together with Nefer. (In the provided Youtube Video, it starts around the 2:54:00 mark)
Dori shows the Traveller a Device that her “supplier” made for her. In that conversation, we learn that Kaveh had been asked to create the device after Nahida asked him for it. In true Kaveh fashion, he created multiple designs of the Device and stressed over it for a while.
He delivered the Device with the help of Mehrak together with a letter that asks her about legal questions. Dori theorizes that these Questions were a favor for Alhaitham. Especially since he has done something similar before.
The device turns out to be crucial in defeating Rerir, as it is used by Albedo and Durin to convert Rerir’s power and Channel it into Columbina.
Nod-Krai Act 7:
In Act 8 of the Nod-Krai Archon quest, Albedo marvels over the design of the device, thinking it was Nahida that created it, when it was in fact, Kaveh.
As the twitter user @ukekaveh pointed out, Albedo mentions that the Device Kaveh built uses light refractions and prisms, which could be a reference to Alhaithams attacks and motifs of Reflection and light. This could therefore mean, that Alhaitham helped in building the device together with Kaveh.
