How Monster Research (MR) works
Monster Research (MR) is Ragnarok Origin: Classic's long-term account progression: by defeating monsters you level up their research and accumulate permanent account MR. This guide explains the mechanics and how to farm efficiently.
Element lines
Every element (Water, Fire, Earth, Wind, Poison, Holy, Shadow, Ghost, Undead, Neutral) has lines of 5 monsters. Each monster has its own research level from 0 to 10, raised by defeating it. Higher research on a monster gives more of its stat bonus.
The line bonus (the key)
A completed line grants a line bonus based on the lowest research level among its 5 monsters. In other words, a single under-leveled monster caps the whole line's bonus. This "bottleneck" is what you usually want to raise first — pushing the weakest monster of a line up a level can unlock a big cumulative jump.
Research per kill
Each kill gives research, but the amount per kill decreases as the monster's research level rises (early levels are cheap, later levels take many more kills). That means a fresh monster at level 0 is far more "MR per kill" efficient than one already near 10.
Picking the best map to farm
The best AFK farming map is the one where you can hitkill everything and still gain a lot of research per kill. Low-level maps are usually best early (weak monsters = easy hitkill), and a map stops being worth it once all its normal monsters are maxed. That's exactly what the ROOC Companion planner computes for you.
MVPs and Minis
MVP and Mini monsters have their own research too. MVPs are hunted actively (they can kill you in AFK and respawn every hour), while Minis can kill a weak character and slow a map down for a strong one. A line-locking MVP that is the sole lowest of a complete line can be worth a huge amount of MR per kill, because it unlocks both its own bonus and the line bonus.
Using ROOC Companion
The planner tracks your MR per monster, tells you the best map to farm right now (and the best MVP to hunt), and recommends an element converter. Everything is saved in your browser. Browse the monster database, the maps and the cards for stats, spawn info and where each card drops.
Typing all of that in is the tedious part. 
Companion Live is a free Windows program that reads the game's connection on your PC and fills the planner from the kills you actually made, character by character. Its page explains what it reads, what it never does, and the risk of using it — read that before you install.
The monster page answers with your number, not an average: with the planner filled in it shows what the kill pays at your research level — including the slice of the line bonus, when that monster is the bottleneck — and says in one word whether it is worth hunting now.