The reason MISSINGNO. is who he is:

In Red and Blue version, the base stats starting offset is 0x0383DE. To find any pokemon, you take 0x0383DE + (PkmnNo. − 1) × 0x1C, because 0x1C is the size of the base stats structure. MISSINGNO. has clear data pointing him to pokemon number 0. When 1 is subtracted from this, it underflows to 0xFF, causing it to read data from 0x0383DE + 0xFF × 0x1C, or 0x39FC2. The data (0x1C bytes) located here is as follows:

`002188001D0606001D8F880019378F37378F001A37370D37001C0D0D`

This data, however, is trainer data; specifically, data for Bikers.

Byte Value Meaning for MISSINGNO. Meaning in the actual game data
0x00 Coincidentally, the first byte, which is supposed to be the pokemon’s number, happens to be 0. Byte 0x00 is a seperator for different trainers. Specifically, this one means Biker 0x08’s data is over.
0x21 Base HP of 33. The level of the one Pokemon Biker 0x09 has is 33.
0x88 Base Attack of 136. The one Pokemon Biker 0x09 has is Muk, whose internal id is 0x88.
0x00 Base Defense of 0. Once again, this byte means Biker 0x09’s data is over.
0x1D Base Speed of 29. Both Biker 0x0A’s Pokemon are level 29.
0x06 Base Special 6. Biker 0x0A’s first Pokemon is Voltorb, whose internal id is 0x06.
0x06 Type 1 is Bird. Biker 0x0A’s second Pokemon is Voltorb, whose internal id is 0x06.
0x00 Type 2 is Notmal. Once again, this byte means Biker 0x0A’s data is over.
0x1D Catch rate of 29. Somewhat difficult to catch I guess. Both Biker 0x0B’s Pokemon are level 29.
0x8F Base Exp. Yield of 143. Dunno how good that really is. Biker 0x0B’s first Pokemon is Weezing, whose internal id is 0x8F.
0x88 Dimensions of frontsprite is 8x8 blocks (or 64x64 pixels). This doesn’t appear to actually affect the game, however, because the game only allows a maximum of 7x7 blocks (56x56 pixels). Biker 0x0B’s second Pokemon is Muk, whose internal id is 0x88.
0x00 Pointer to frontsprite is at RAM address 0x1900 (reverse‐byte order). The first 0x4000 bytes of a ROM and a RAM dump line up. How this affects the picture, I’m unsure, only that the bank loaded has absolutely nothing to do with the image loaded, which is why every MISSINGNO. looks the same despite the differences in internal id numbers. Once again, this byte means Biker 0x0B’s data is over.
0x19 All of Biker 0x0C’s Pokemon are level 25.
0x37 Pointer to backsprite is at RAM address 0x8F37. Not sure what this data is. Biker 0x0C’s first Pokemon is Koffing, whose internal id is 0x37.
0x8F Biker 0x0C’s second Pokemon is Weezing, whose internal id is 0x8F.
0x37 First wild move is Water Gun, or move 0x37. Biker 0x0C’s third Pokemon is Koffing, whose internal id is 0x37.
0x37 Second wild move is Water Gun, or move 0x37. Biker 0x0C’s fourth Pokemon is Koffing, whose internal id is 0x37.
0x8F Third wild move is Sky Attack, or move 0x8F. Biker 0x0C’s fifth Pokemon is Weezing, whose internal id is 0x8F.
0x00 There is no fourth wild move. Once again, this byte means Biker 0x0C’s data is over.
0x1A How much experience is needed to level up. Read about it here. The only valid numbers for this is 0, or 3–5, so I have no idea how this affects training of a MISSINGNO. All of Biker 0x0D’s Pokemon are level 26.
0x37 TM and HM flags. If you really care enough, read the bulbapedia article I mentioned early, and convert to binary and figure out what TMs and HMs it can learn, because I’m too lazy to do it myself or copy/paste from someone who’s alreay done it. Biker 0x0D’s first Pokemon is Koffing, whose internal id is 0x37.
0x37 Biker 0x0D’s second Pokemon is Koffing, whose internal id is 0x37.
0x0D Biker 0x0D’s third Pokemon is Grimer, whose internal id is 0x0D.
0x37 Biker 0x0D’s fourth Pokemon is Koffing, whose internal id is 0x37.
0x00 Once again, the next byte means Biker 0x0D’s data is over.
0x1C All of Biker 0x0E’s Pokemon are level 28.
0x0D Biker 0x0E’s first Pokemon is Grimer, whose internal id is 0x0D.
0x0D Biker 0x0E’s second Pokemon is Grimer, whose internal id is 0x0D.

# Resources used

Besides the aforementioned bulbapedia article, I used this page on the ultimate pokemon center site, this hex list from TRsRockin site, and a ROM map on datacrystal provided by none other than Coolboyman.