Getting a master
You can get a master by pressing the button labelled as '1' - Invite Master. You can only get one master at any one time. The benefit of having a master is that you get experience and drop rate buffs. There are two buffs, one of which is static (never turns off) and one is active in a party.
Risingblade (the apprentice) receives these two buffs. The experience+5% one is static, but has no drop rate. This will always be active. The other is 5% exp and 5% drop rate, which is active when in a party with your master.
These party buffs scale depending on your master's level.
Currently, there are 3 levels:
Level one: "Elementary master" gives 5% exp and 5% item drop rate. Their static buff gives 5% exp.
Level two: "Intermediate master" gives 10% exp, 10% item drop rate. Their static buff gives 10% exp.
Level three: "Advanced master" gives 15% exp, 15% item drop rate. Their static buff gives 15% exp.
You can see your master's level in the "Master information" box.
You can leave a master at any time by pressing the button labelled '2.' - Master farewell.
Getting apprentices and increasing your master level
After you are level 40, you can recruit apprentices by pressing the button labelled '4' - Recruit apprentices. People can also request to be your apprentice by right clicking you, and then hitting "Seek Apprenticeship." You or them will then have to confirm that they can be your apprentice.
Getting apprentices doesn't really have any benefits - it just gives them buffs when you're online or in a party with them. It does give you rewards when your apprentice levels up, though. Also, by doing challenges with your apprentices and master, you can increase your master points. Your master points are on the interface as shown above, labelled "Masters: XX". In mine, it says "Masters: 75", because I have 75 master points.
Please note that the amount of apprentices you can recruit varies with level.
Level 40: You may only have 4 apprentices.
Level 55: You may have 5 apprentices.
Level 70: You may have 6 apprentices.
Level 85: You may have 7 apprentices.
Level 100: You may have 8 apprentices.
Levelling up as a master
Elementary Master: This is the first level. You automatically become an elementary master when you're level 40 and you have an apprentice.
Intermediate Master: This is the second level. You are required to reach 500 master points to reach this level along with at least two apprentices. When achieving it your buffs will upgrade, as said above. (See "Getting a master.")
Advanced Master: This is the last level. You are required to reach 2000 master points and have at least 4 apprentices. Again, your buffs with upgrade. (See "Getting a master.")
When you've reached the specified amount of master points and apprentices, you can level up by going to Jale NPC <Maestro> Mashalt. Location X:125, Y:256.
These are shown by clicking the button labelled '6' - Level up rewards. When the apprentice levels up, both the master and the apprentice will earn the corresponding to that level. Also, when the apprentice levels up, the master will gain master points. (Also corresponding to the level.)
Master points based on level:
Level 15-30: +1 Master point per level.
Level 31-40: +2 Master points per level.
Level 41-50: +4 Master points per level.
Level 51-60: +8 Master points per level.
Level 61-70: +12 Master points per level.
Level 71-80: +20 Master points per level.
Level 81-90: +40 Master points per level.
Level 91-100: +100 Master points per level.
Mentor certificates from bags
As mentioned, at certain levels, the master and apprentice will receive bags as a reward. In these bags, there are various mentor certificates that you receive. These can be handed in at the Jale NPC <Senior Mentor> Raymin when you have <Mentor Certificate>x20, and you will receive a "Master's fortune bag" for handing them in.
Jale NPC, <Senior Mentor> Raymin. Location X:125, Y;258.