Miror. B is one of the Cipher Admins from 5 years ago. However since then he has been kicked out of Cipher and has become a common thief. You will encounter him a few times during the game but he does have an extra purpose
At any time whilst walking around in Orre you may find an alert come up. This is your Miror. B radar. It will tell you where he is and you go find him. He can be anywhere, at any of the PokéSpots, in any Town, Any Stadium Now the extra feature he has is that if you miss a Shadow Pokémon from a trainer who leaves their position (Such as Cipher Admins, Random Cipher Peons and generally everyone who doesnt stick in one place), he will have stolen the Shadow Pokémon from them and will battle you with them as many times as necessary in order for you to Snag them.
Once you have snagged all 83 Shadow Pokémon either from him or from everyone else. He'll destroy your Miror B Radar. Below is a list of Shadow Pokémon you can get from him

Picture	No.	Name	Level	Original Trainer Name	Gender
	#167	Spinarak	Lv. 14	Cipher Peon Kishirin	50% Male ♂ 50% Female ♀
	#322	Numel	Lv. 14	Cipher Peon Ayuya	50% Male ♂ 50% Female ♀
	#285	Shroomish	Lv. 15	Researcher Heliko	50% Male ♂ 50% Female ♀
	#318	Carvanha	Lv. 15	Cipher Peon Hilga	50% Male ♂ 50% Female ♀
	#301	Delcatty	Lv. 18	Cipher Admin Lovelina	25% Male ♂ 75% Female ♀
	#100	Voltorb	Lv. 19	Wanderer Miror.B	Genderless
	#296	Makuhita	Lv. 18	Cipher Peon Gorus	50% Male ♂ 50% Female ♀
	#037	Vulpix	Lv. 18	Cipher Peon Feres	50% Male ♂ 50% Female ♀
	#355	Duskull	Lv. 19	Cipher Peon Siam	50% Male ♂ 50% Female ♀
	#280	Ralts	Lv. 20	Cipher Peon Fergus	50% Male ♂ 50% Female ♀
	#303	Mawile	Lv. 22	Cipher Peon Dakacha	50% Male ♂ 50% Female ♀
	#361	Snorunt	Lv. 20	Cipher Peon Kyaris	50% Male ♂ 50% Female ♀
	#204	Pineco	Lv. 20	Cipher Peon Bimol	50% Male ♂ 50% Female ♀
	#177	Natu	Lv. 22	Cipher Peon Romosh	50% Male ♂ 50% Female ♀
	#315	Roselia	Lv. 22	Cipher Peon Shizu	50% Male ♂ 50% Female ♀
	#052	Meowth	Lv. 22	Cipher Peon Fese	50% Male ♂ 50% Female ♀
	#220	Swinub	Lv. 22	Cipher Peon Yorin	50% Male ♂ 50% Female ♀
	#021	Spearow	Lv. 22	Cipher Peon Annon	50% Male ♂ 50% Female ♀
	#088	Grimer	Lv. 23	Cipher Peon Dagu	50% Male ♂ 50% Female ♀
	#086	Seel	Lv. 23	Cipher Peon Gowan	50% Male ♂ 50% Female ♀
	#337	Lunatone	Lv. 25	Cipher Admin Wazzle	Genderless
	#299	Nosepass	Lv. 26	Wanderer Miror. B	50% Male ♂ 50% Female ♀
	#335	Zangoose	Lv. 28	Bodybuilder Zakusuka	50% Male ♂ 50% Female ♀
	#046	Paras	Lv. 28	Cipher Peon Niuri	50% Male ♂ 50% Female ♀
	#058	Growlithe	Lv. 28	Cipher Peon Niuri	75% Male ♂ 25% Female ♀
	#090	Shellder	Lv. 29	Cipher Peon Hoshu	50% Male ♂ 50% Female ♀
	#015	Beedrill	Lv. 30	Cipher Peon Rigon	50% Male ♂ 50% Female ♀
	#017	Pidgeotto	Lv. 30	Cipher Peon Rigon	50% Male ♂ 50% Female ♀
	#114	Tangela	Lv. 30	Cipher Peon Tarugu	50% Male ♂ 50% Female ♀
	#012	Butterfree	Lv. 30	Cipher Peon Tarugu	50% Male ♂ 50% Female ♀
	#082	Magneton	Lv. 30	Cipher Peon Waguchi	Genderless
	#049	Venomoth	Lv. 32	Cipher Peon Ouda	50% Male ♂ 50% Female ♀
	#070	Weepinbell	Lv. 32	Cipher Peon Ouda	50% Male ♂ 50% Female ♀
	#024	Arbok	Lv. 33	Cipher Peon Suguru	50% Male ♂ 50% Female ♀
	#057	Primeape	Lv. 34	Cipher Admin Gorigan	50% Male ♂ 50% Female ♀
	#097	Hypno	Lv. 34	Cipher Admin Gorigan	50% Male ♂ 50% Female ♀
	#055	Golduck	Lv. 33	Sailor Baruko	50% Male ♂ 50% Female ♀
	#302	Sableye	Lv. 33	Sailor Baruko	50% Male ♂ 50% Female ♀
	#020	Raticate	Lv. 34	Chaser Ankura	50% Male ♂ 50% Female ♀
	#085	Dodrio	Lv. 34	Chaser Ankura	50% Male ♂ 50% Female ♀
	#083	Farfetch'd	Lv. 36	Cipher Admin Lovelina	50% Male ♂ 50% Female ♀
	#334	Altaria	Lv. 36	Cipher Admin Lovelina	50% Male ♂ 50% Female ♀
	#115	Kangaskhan	Lv. 36	Cipher Peon Punto	0% Male ♂ 100% Female ♀
	#354	Banette	Lv. 37	Cipher Peon Punto	50% Male ♂ 50% Female ♀
	#126	Magmar	Lv. 36	Cipher Peon Jama	75% Male ♂ 25% Female ♀
	#127	Pinsir	Lv. 35	Cipher Peon Jama	50% Male ♂ 50% Female ♀
	#219	Magcargo	Lv. 38	Cipher Peon Chisba	50% Male ♂ 50% Female ♀
	#078	Rapidash	Lv. 40	Cipher Peon Chisba	50% Male ♂ 50% Female ♀
	#106	Hitmonlee	Lv. 38	Cipher Peon Petro	100% Male ♂ 0% Female ♀
	#107	Hitmonchan	Lv. 38	Cipher Peon Karuban	100% Male ♂ 0% Female ♀
	#108	Lickitung	Lv. 38	Cipher Peon Lefur	50% Male ♂ 50% Female ♀
	#123	Scyther	Lv. 40	Cipher Peon Preama	50% Male ♂ 50% Female ♀
	#113	Chansey	Lv. 39	Cipher Peon Preama	0% Male ♂ 100% Female ♀
	#338	Solrock	Lv. 41	Cipher Admin Wazzle	Genderless
	#121	Starmie	Lv. 41	Cipher Admin Wazzle	Genderless
	#125	Electabuzz	Lv. 43	Cipher Admin Aldos	75% Male ♂ 25% Female ♀
	#277	Swellow	Lv. 43	Cipher Admin Aldos	50% Male ♂ 50% Female ♀
	#143	Snorlax	Lv. 43	Cipher Admin Aldos	87.5% Male ♂ 12.5% Female ♀
	#062	Poliwrath	Lv. 43	Cipher Admin Gorigan	50% Male ♂ 50% Female ♀
	#122	Mr. Mime	Lv. 43	Cipher Admin Gorigan	50% Male ♂ 50% Female ♀
	#051	Dugtrio	Lv. 40	Cipher Peon Stron	50% Male ♂ 50% Female ♀
	#310	Manectric	Lv. 44	Cipher Admin Eldes	50% Male ♂ 50% Female ♀
	#373	Salamence	Lv. 50	Cipher Admin Eldes	50% Male ♂ 50% Female ♀
	#105	Marowak	Lv. 44	Cipher Admin Eldes	50% Male ♂ 50% Female ♀
	#131	Lapras	Lv. 44	Cipher Admin Eldes	50% Male ♂ 50% Female ♀
	#149	Dragonite	Lv. 55	Wanderer Miror. B	50% Male ♂ 50% Female ♀