Ueki-loid (植木ロイド) is a Japanese VOCALOID licensed to Music Airport Inc., and as a "private" VOCALOID he is not commercially distributed.
The Ueki-loid is based upon the vocals of a famous deceased Japanese comedian, actor, singer and guitarist named Hitoshi Ueki (植木 等 Ueki Hitoshi), who died in 2007 and is a part of the "Legend of VOCALOID" line-up.
5 years before his unveiling, Yamaha was approached with the aim of creating a voicebank based on the voice of the late singer Ueki Hitoshi. Ueki-loid's production began in late 2007. While work was being made on the vocal voicebank library, work was neither confirmed or denied by both parties.
New technology allowed the phonemes to be extracted from recorded vocal tracks in order to create the Ueki-loid. A sample can be heard here.
Despite that it has been declared he will never be released, an album will be released in the future to show off the technology behind the vocals. This is more about promoting the technology that made this possible than the vocal behind the songs.
The voicebank was developed as follows: First, they made a voicebank from the voice of Hiro Kouichi (surname first), the eldest son of Ueki Hitoshi. Then, by a certain mathematical method, they constructed a transformation function which represented the difference in voice between the two people. Finally, they applied the function to the voicebank.
It was revealed that the VOCALOID is for private use only and will never be released.
The vocal itself overall has a VOCALOID engine quality to it.
Since much of the phonetic data was impossible to retrieve, as they were never recorded for Ueki Hitoshi in the first place, what you hear is mostly the VOCALOID™ engine between lyrics. Still, despite this, a lot of people were pleased with the voice and stated that it sounds similar to Ueki Hitoshi.
Since the original VOCALOID software had been released, it had been hoped that the voices of lost singers like Elvis could be brought back to sing again. Ueki-loid is the first step in that direction, although it has been noted it will be a while before they can attempt a English voicebank using the same techniques.
Some sources say he is the first private announced Vocaloid, however, this is not true. A Vocaloid-Flex voicebank was used in the game Metal Gear Solid: Peace Walker, also many private proto-type vocals also have been known to exist for Vocaloid with no indication any of them will ever be released. Even with this not taken into account, the first private vocal confirmed was Akikoloid-chan, who was revealed on September 6th, 2011, a few months prior to Ueki-loid's announcement.