Hello, my name is K. I was in charge of organizing this year's conference. As announced in the LINE DEVELOPER DAY_2015 Tokyo Announcement « LINE Engineers' Blog post, the LINE DEVELOPER DAY_2015 conference was held on April 28th, 2015.
There were so many people that signed up for the conference, we could not invite everyone. But thanks to all the engineers that showed interest in the event, the conference was a huge success. We would like to express our sincere gratitude! 
I was in charge of planning and running the event, including the minute details. Planned as an opportunity to "let engineers outside the company have a deeper understanding of LINE," it was certainly not an easy task. But the event was well received both during and after the conference, even trending on Twitter. We sincerely hope we can meet you all again with another conference in the near future. 
In-depth reports on the event are still in preparation. But in the meantime, here are the videos taken during the conference along with the presentation slides.
Opening Remarks Chief Executive Officer Takeshi Idezawa
LINE Global Culture Chief Technology Officer Euivin Park
LINE Messenger for the World Senior Executive Officer, Head of Service Development Ikebe Tomohiro
LINE Platform Development Chronicle Tom.T
Slides can be found here→ LINE Platform Development Chronicle
HBase and Redis: LINE's Storage System for Processing Over 100 Million Messages Per Day Shunsuke.N
Slides can be found here→ HBase and Redis: LINE's Storage System for Processing Over 100 Million Messages Per Day
Evolution of and Challenges in Developing LINE Android over the Last 4 Years Tsutomu.H
Slides can be found here→ Evolution of and Challenges in Developing LINE Android over the Last 4 Years
Burgeoning Sticker and Theme Sales Systems: A Chronicle of Risks and Revival Haruki.S
Slides can be found here→ Burgeoning Sticker and Theme Sales Systems: A Chronicle of Risks and Revival
How Should LINE Creators Market have been Developed Amidst the Internationalization of Web Services? Tokuhiro.M
Slides can be found here→ How Should LINE Creators Market have been Developed Amidst the Internationalization of Web Services?
Replacement of LINE’s Messaging Pipeline with Akka Actor and AMQP Yuichi.O
Slides can be found here→ Replacement of LINE’s Messaging Pipeline with Akka Actor and AMQP
LINE for Apple Watch Christopher.R
Slides can be found here→ LINE for Apple Watch
Changes to the Development Team Due to the Implementation of Swift Yosuke.I
Slides can be found here→ Changes to the Development Team Due to the Implementation of Swift
Development Strategies for a Global Network Environment and Multi-OS Supporting LINE Game Client Platform Heejun.K
Slides can be found here→ Development Strategies for a Global Network Environment and Multi-OS Supporting LINE Game Client Platform
TV TOKYO x LINE Business Connect Sato Nobuaki of TV Tokyo Communications Corporation
Slides can be found here→ TV TOKYO x LINE Business Connect
Platforms for Analyzing Big Data: Analytical Techniques Required to Aggregate Data Taiichi.H
Slides can be found here→ Platforms for Analyzing Big Data: Analytical Techniques Required to Aggregate Data
Developing Recommendation Engines that Utilize Bayesian Inference and Deep Learning Jun.N
Slides can be found here→ Developing a Recommendation Engine Based on Bayesian Inference and Deep Learning