Firstly, Ashina: The Red Witch has no alternative endings as the game is a prequel and there wouldn't be a logical way to continue the story if otherwise, hence why I made an unlockable extended ending which is all canon.

How each ending unlocks works on a point system. Throughout the game, you will hear a small bell sound once you have earned a point (similar to the save sound). These points accumulate towards a specific ending.

I would also advise making multiple saves throughout the game especially in the sewers as one crucial point relies on starting the sidequest before you leave.

Ashina Ending:

To obtain the Ashina ending, just complete the game as normal, it will unlock regardless of how you play through it.

Tena Ending:

• Talk to Tena about her dream in Ash's apartment during the prologue dialog.
  
• Talk to Tena about Yoko in the spirit world bathhouse.
  
• Check Tena's bath before leaving the spirit world bathhouse with Tanto.
  
• Check the sauna in the final chapter bathhouse for Tena.

Bonus Ending:
(You don't need all of these to unlock just most of them)

• Check the geisha doll statue in the buffet of the witch's house.
  
• Interact with Luzia's bed in the witch's house multiple times.
  
• Talk to Nura in the bath in the witch's house.
  
• Give the Forest Spirit a [Bowl of Noodles].
  
• Check the Stranga mask in the sauna in the final bathhouse.
  
• Talk to Seki about the dark tunnels in Kozan.
  
• Talk to Kusari about who is the witch.

Secret Ending:
(To unlock this ending you will need to do all tasks)

• Give the [Bun] to Oakie after exhausting his dialog.
  
• Give the talking Hoki the [Chocolate] in the middle houses in Kozan.
  
• Give the [Tuning Fork] to the chef stirring the pot of soup in the witch's house after using it.
  
• Give Kusari the [Bowl of Cod Soup].
  
• Give the [Bag of Rice] to the Kappo at the sewer entrance in the inner city of Chikara.
  
• Give the [Mail] to Post Ninja in Chikara residential district.
  
• Give Gus the [Photo].
  
• Give Kubi the [Old Photo].
  
• Give Tanto the [Old Photo]. (This will be done automatically if in inventory when seeing Tanto)
  
• Exhaust Noryo's dialog options as Ash in Chikara residential area.

Obtain Special Items
There are some items needed in order to achieve some of the endings.

Mail:
To obtain the mail, you can find it behind the temple in Chikara

Bag of Rice:
You can obtain the bag of rice from Pretty Woman in the Chikara Residential District, you will need coins to pay for it.

Old Photo:
In the chapter "Road To Nowhere", you can find "Old Photo" behind a tree where you meet Gus.

Photo:
You can obtain the "Photo" by exhausting Kubi's dialogue