Summary of the Willy Report
The suspicious user accounts identified by the Willy Report all followed a particular pattern:
Each account was active for a single time period, and only one account was active at a time.
Each account bought 10-20 bitcoins every 5-10 minutes.
Each account only bought bitcoin with USD, and never sold any coins.
Each account bought bitcoin up to a very specific total USD amount (e.g. 2,500,000 USD).
After each account was "finished", another account would shortly become active and continue buying.
These accounts were seemingly able to trade even when MtGox was inaccessible to the world.
All trades by these accounts had unusual data in the trade log files.
This automated trading began on September 27, 2013 and continued at least until the end of the leaked log data (at the end of November 2013). After this point there is no trade data available publicly, but people claim to have observed this behavior continue into 2014, and supposedly at some point after that, the reverse started happening; automated selling of bitcoin at regular intervals.
The second half of the Willy Report investigates another suspicious account in the leaked trade logs, with seemingly incorrect fiat amounts recorded for its trades. This account exhibited different behavior but like Willy it seemed to buy a lot of bitcoin during 2013 (February through September) only to suddenly stop, mere hours before the first Willy account began trading. The report dubbed this user "Markus", and later concluded that it was using the MtGox trading account of CEO Mark Karpelès, though the log data was inconsistent and may have been intentionally manipulated specifically to conceal or obfuscate this account activity. Ultimately the relationship between Markus and Willy remains unclear.
Impact on the MtGox market
Willy bought a very large amount of bitcoin on MtGox during the period of September 27 November 30 during 2013 (and later, though the leaked logs end on this date), a total of over 250,000 BTC. There is a very high probability that this had a large effect on the price of bitcoin, opening up the possibility that this may have been a plan to manipulate the market rather than (or in addition to) fraudulently acquiring bitcoins. Another speculation has been that MtGox for some reason had a shortage of bitcoins and used their own exchange to acquire more, trading BTC shortage for USD shortage.
To get an overview of just how significant this activity was, the following is a graph of how much of the hourly trade volume on MtGox was actually Willy, with the MtGox bitcoin price overlaid:
As clearly seen, for a lot of the time (especially when the market was otherwise quiet), Willy had a significant presence, and it is hard to think that this would not have an effect on the market and in turn the exchange price, through its added buying pressure. There are even some suspicious incidents where Willy becomes absent and soon afterwards the market "corrects" itself to a lower price level.
A question we cannot answer without more data is what continued influence Willy had after November 30. Certainly there were additional strong price climbs later that in the light of this might now appear suspicious, but it is also a fact that the price never peaked past its high point at the end of November. Did Willy stop trading, or was the amount of bitcoins it bought no longer large enough to keep pushing the price even higher? Did Willy or similar fraudulent trading play any role in the massive price crash that began in early February? And finally, what happened to all the bitcoins that Willy seemingly amassed?