Boktai is a video game series developed by Konami for the Game Boy Advance and Nintendo DS handheld consoles. The title is an abbreviation of the series' full Japanese title Bokura no Taiyo or Our sun. They are recognized for using a solar sensor that is a key element of gameplay. The Boktai games are produced by Hideo Kojima, creator of the Metal Gear series, who also came up with the initial game design and concept.
The first game uses a major attachment onto the game's cartridge, a solar sensor. This has major effects in gameplay, and is a key element needed to progress. The amount of ultra-violet rays that the sensor detects is displayed on the bottom right-hand corner of the screen. The sunlight is measured in blocks, with a maximum of eight blocks. The main character, Django, fights with a solar-powered gun known as Gun Del Sol (Solar Gun), which uses batteries which store energy, displayed at the bottom right-hand corner. This is solar energy collected via the sensor or various methods in gameplay. This energy can be stored in solar stations which the player can withdraw and use when sunlight is not available. The solar energy in a solar station is limited to how much the player has collected. The system of storage also uses the amount of energy in a battery. Also, solar energy can be stored in the solar bank to gain interest and pay debt. The solar bank can store the same amount of energy as the solar stations. Using stealth and Django's solar gun, the player can purify enemies known as boks, or undead. These enemies are found in various dungeons located in Istrakan, the City of Death, throughout the game. Boktai has a built in clock, which enables the game to adjust to different times of the day. This also affects gameplay, because the environment changes with the clock, and different enemies may be active or dormant.
The second game continues to use a solar sensor and a built in clock, but this time the amount the sensor can detect was increased to ten blocks. Unlike the first game, Boktai 2 is more of an Action RPG than a stealth game. Nevertheless, keeping the same qualities in Boktai, Hideo Kojima expanded characters and maps, enemies and weapons. A new feature, Solar Forging, allows the player to forge two weapons together to create a new, stronger weapon, sometimes with special abilities. The player may unexpectedly end up with a very special weapon, known as a 'R-Rank' weapon. During gameplay, the storyline temporarily shifts the player character to Sabata, who is Django's older brother.
The third game in the series, released only in Japan, also uses the solar sensor, but it has changed its gameplay mechanics slightly. The main weapon is still the Gun Del Sol, but instead of attaching different frames, lenses, batteries, and grenades, only lenses and frames are changed; plus, hammers and spears have been removed from the game, using swords instead. It also does without the overworld map seen in Boktai and Boktai 2, replacing it with a stage-select theme. Also, the Casket Cycle was introduced; this requires the player to go through a race-like event when first going to a dungeon, and also allows for Casket Cycle races between players over link mode. Shin Bokura no Taiyō also features a "Crossover Battle 2" with Rockman.EXE 6 (the Japanese version of Mega Man Battle Network 6).
After skipping the third game, Kojima Productions decided to localize the next game, Bokura no Taiyō: Django & Sabata (or Boktai DS) as an unrelated product. The main characters, Django and Sabata, were renamed Aaron and Lucian respectively. According to producer Kensuke Yoshitomi, this was done to distance the fourth game from the earlier Boktai games, due to the lack of a solar sensor.